Almost had Dan Didio out last year, but he had to withdraw at the last minute in light of the Marvel/Disney and Time-Warner/DC shakeups, so we got the awesome Greg Rucka and Matthew Clark, as well as the gorgeous Nicola Scott and legendary Bill Sienkewitz and Peter David.
-Chris (Dying to hear this year's announcements)
chaoticgeektwitter "And therefore as a stranger give it welcome. There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy." Hamlet Act 1, scene 5, 159–167
There is no single answer to that. Most guests are quite busy simply signing...etc at Armageddon and have no time to look at Portfolios. Others have made it clear they ARE interested. The only thing you can do is ask Bill closer to the event to forward a messsage about this to the guest in question.
For the best response however, you need to contact them yourselves and arrange something. Otherwise I'd suggest talking to local artists who have cracked the US market like Nicola Scott (a regular Armageddon comic guest), and find out how they went about it.
Sadly there haven't been any 'official' portfolio reviewers or anything at Armageddon. (In 2007 there was going to be a portfolio review and comp. with David Wohl but for some reason it was all scrapped.) I suggest asking a few of the guests before-hand if they'd mind taking a look at your portfolio at the Con. Although they most likely wouldn't be able to get you were even if your stuff was amazing they will have heaps of helpful advice. In 2008 I asked Jim Lee before the Con (got in contact with him via Myspace and we'd talked a little before this) if he wouldn't mind taking a look in my portfolio and he agreed. On the Sunday I waited in the line with people getting sketches from him and after 6 hours it was finally my turn; in the end we met up after the Con and he gave me 45 minutes of his time to look over my stuff. It was great, didn't get any work out of it but got some of the best advice ever and a really amazing experience.
This may not happen to everyone but it's worth a try.