Both Myers and Dawkins had registered for the event using their real names, so it seems likely that someone just happened to see Myres name and decided he should be kept out, but didn't happen to notice Dawkins' name.
Edit: it turns out that Dawkins' name was not on the list as he was registered only as Myers' unnamed guest. Myers booked the seats on a website where it was not required to name guests. Presumably, then the organisers spotted Myers' name on the list, but had no reason to suspect Dawkins would also be there.
This story is already widely reported and it is to be hoped that the screening's organisers have shot themselves in the foot publicity-wise. This is especially apparent as the sordid details of the film's history and what went on at the screening emerge.
To begin with, Myers and Dawkins appeared in the movie only because they were told that it was to be about the interface between science and religion and would have the more innocuous title 'Crossroads'. Plainly, this was a gross and presumably deliberate distortion (I would go as far as 'deception').
Secondly, some of the questions asked were disingenuous at best. For example, according to PZ Myers' son, Dawkins was asked to put a number on his confidence that evolution was true. He explained that it wasn't appropriate to assign a simple number to something like that, but when pressed suggested 99%. This was obviously intended as a trap, since the interviewer started disputing how Dawkins could come to that particular number (99? Why not 97? Or 94?). Dawkins answered by saying "well....you asked me to put a number on it...." And was promptly ridiculed. This is a standard creationist tactic: cast a scientist as ridiculous and use that to imply that evolution is wrong. Since evolution is wrong, ID must be right, yes? It is about the most pathetic and excruciating types of argument it is possible to come up with.
Dawkins also explained that a designer would itself have to have been designed or evolved, so the only way to bottom out the infinite regress is for the final, ultimate designer to be a product of evolution. He added that if evidence were found for ID on Earth, it would imply that the designer was an alien rather than a god. Again, this review was ridiculed: "Ha ha ha, Dawkins thinks aliens are more likely than god!" Well....yes. They are. We know that life appeared at least once, but there is no evidence that a god did. This is again utterly disingenuous if not downright dishonest. It is a similar type of argument to demanding that Darwin reveal which of his grandparents was a monkey.
The screening was apparently part of a stealth campaign, preaching literally to the choir in order to get the word out. This in itself seems more than a little dishonest: if the film stands or falls on its evidence, why not trumpet its launch, as with any other movie? Especially as it was being released at Easter, of all times. It's a marketing ploy and an unsubtly dishonest one. According to this source:
They postered the Orlando Sentinel with email invitations, then tried to withdraw the one they sent to me. No dice. They also passed out non-disclosure "statement of confidentiality" agreements for people to sign. I didn't.Recording devices were forbidden at the meeting on threat of a $250,000 fine(!)
I should point out that I haven't seen Expelled yet. I will get around to wasting an hour and a half of my life on it when it comes to a browser near me and I'll sure I'll report on it then.
Edit: it seems that the film's makers (Mark Mathis and Ben Stein) have been trying to edit history by claiming that Myers was expelled because the event was private and Myers wasn't invited. In fact, this was the answer Mathis gave at the screening's Q&A session, when Dawkins asked why Myers had been expelled. The answer seemed to satisfy the audience despite the fact that it was a complete lie: attendance at the event was by registration on a web site as every single person there presumably knew. Creationists have subsequently claimed that Myers and Dawkins were gatecrashers at the screening, which is a simple, straightforward lie. See here for example. Dawkins does indeed 'admit' that he wasn't invited (other than by Myers) and didn't have a ticket, but of course neither had/did anyone else. The article is not content with blatant lying though: it also uses this lie to ridicule what it calls Dawkins' 'ideology'. In other words, an ad hominem attack on all evolutionists. I wish I could say this was less common amongst the ID crowd.
Dawkins' account (from which most of the edits come) can and should be seen here.
A long list of blogs with posts on this topic can be found here: http://scienceblogs.com/gregladen/2008/03/pz_myers_expelled_gains_sainth.php.