So I'm reading Charles E. Hill's essay "Ignatius, 'the Gospel' and the Gospels" in Gregory & Tuckett's The New Testament and the Apostolic Fathers: Trajectories through the New Testament and the Apostolic Fathers (amazon.com). In Hill's discussion of ISm 5.1, there it is:

Gundry argues that the strong adversative, αλλα, just before 'the gospel' distances it from the law and the prophecies and aligns it with 'our human sufferings'. But any distinction intended with the adversative is surely temporal (note μεχρι νυν). (Hill, in Gregory & Tuckett (amazon.com) 277-278).

Hill is arguing that at least some of Ignatius' uses of ευαγγελιον are in reference to a written gospel. Gundry, whose work I'm not familiar with (R. Gundry, 'ΕΥΑΓΓΕΛΙΟΝ: How Soon a Book', JBL 115 (1996), 321-5; I'll have to check out the article), is arguing the opposite, at least in this case.

My eyes and ears perk up when I see something attributed to αλλα, the 'strong attributive'. So we can all be on the same page, here is the Greek and English of Ign. Smyrn. 5.1:

5.1 Ὅν τινες ἀγνοοῦντες ἀρνοῦνται, μᾶλλον δὲ ἠρνήθησαν ὑπʼ αὐτοῦ, ὄντες συνήγοροι τοῦ θανάτου μᾶλλον ἢ τῆς ἀληθεῖας· οὓς οὐκ ἔπεισαν αἱ προφητεῖαι οὐδὲ ὁ νόμος Μωσέως, ἀλλʼ οὐδὲ μέχρι νῦν τὸ εὐαγγέλιον οὐδὲ τὰ ἡμέτερα τῶν κατʼ ἄνδρα παθήματα·
Holmes, M. W. (1999). The Apostolic Fathers : Greek texts and English translations (Updated ed.) (186). Grand Rapids, Mich.: Baker Books.

5.1 Certain people ignorantly deny him, or rather have been denied by him, for they are advocates of death rather than the truth. Neither the prophecies nor the law of Moses have persuaded them, nor, thus far, the gospel nor our own individual suffering;
Holmes, M. W. (1999). The Apostolic Fathers : Greek texts and English translations (Updated ed.) (187). Grand Rapids, Mich.: Baker Books.

In reading about αλλα (lexicons, grammars, monographs, etc.), in examining every instance in the NT and in the Apostolic Fathers, and in thinking about what αλλα does, my basic conclusion is similar to that of Heckert. I don't see the 'adversative' nature of αλλα functionally separating things, I think αλλα essentially marks a relationship of contrast between two items at the same discourse level (so, words, phrases, clauses, paragraphs, discourses, etc.). The degree of contrast is dependent upon context (vocabulary, other grammatical elements, etc.). Instances of αλλα normally occur with a "not", hence the so-called 'adversative' nature. But even without "not", the contrast is there, you just have to look.

In other words, contra Gundry, I'd say that in Ign. Smyrn. 5.1, αλλα doesn't distance anything from anything else. It denotes that two items are in fact joined for the sake of contrasting them. In this case, "the prophecies and the law of Moses" are contrasted with "the gospel" and "our own individual suffering". Importantly (and most favorable to Hill's argument, I'd say) "the gospel" is being contrasted against "the prophecies and the law of Moses". Ignatius is running through a list of things that, by all rights, should have persuaded those who deny, but haven't. The prophecies haven't, the law of Moses hasn't, the gospel hasn't, and the present suffering of Christians hasn't. As Hill points out, μεχρι νυν ("until now") is the kicker here. We have the past (prophecies and law) in contrast with the present (gospel and suffering).

Additionally, I'd say that in such comparisons, the stuff after αλλα (gospel and suffering) is the more prominent of the material. In other words, I'd say the structure highlights that "the gospel" and "our own individual suffering" haven't even served to persuade these deniers of the truth. While the prophecies and law of Moses should've done the persuading, that isn't really Ignatius' point; his point is that not even up to this point, these people, knowing the gospel (written or not) and seeing our suffering, even now still deny the truth.

In other words, I don't think the use of αλλα has anything to do with separating written content from oral content (in this instance); I think it has to do with Ignatius' amazement that folks could still deny in light of having the gospel and seeing the suffering that professing Christians were willing to endure.

Update I (2008-10-20): Note the comment by Mike Aubrey (of εν Εφεσω). He asks if I see αλλα as only functioning in a coordinating relationships. On your question, Mike, I'd have to say "yes", though I'll note that even Denniston has some examples (as I recall, in his sections on both αλλα and μεν) where αλλα and other conjunctions (even asyndeton) are used to respond to either a general idea ascertainable from the context, or in response to something well before in the discourse. I'd say Mk 16.5-7 (see my previous post on these verses) is an example of stuff like this.

Update II: Also please note that Dr. Carl Conrad, of B-Greek fame, sent along the following note which, for some reason, the commenting feature didn't allow him to post:

Rick, I think you're probably right about this; despite my warnings not to confuse the conjunction ἀλλὰ with the neuter accusative plural pronoun ἄλλα, I rather suspect that the conjunction originated in an adverbial usage of the neuter accusative plural pronoun with a sense "otherwise" — that it became a stronger equivalent of δὲ in μὲν ... δὲ (beginning Greek students are still taught that μὲν ... δὲ means "on the one hand ... on the other hand"). But (ἀλλὰ) there's an interesting idiomatic expression in older Greek using the pronominal adjective ἄλλος/η/ο with a καὶ to underscore the term following the expression (LSJ s.v. ἄλλος II.6) ἄλλοι τε καὶ ἐκεῖνος = "especially 'that one'"; an adverbial use is also not uncommon: ἄλλως τε καὶ (LSJ s.v. ἄλλως I.3) with the sense "especially." My surmise may very well be wrong, but I've long thought that ἀλλὰ derives originally from the adjectival pronoun ἄλλα used in the adverbial accusative.