Yeah, this is one of my favorite points as well. One problem with trying to make it to a creationist is that there are so many different types of creationists. When mentioning allelic diversity in my experience you'll find one of the following responses:
a) a confused creationist that uses both "no beneficial mutations" arguments alongside "no new information" rhetoric and generally hops back and forth between the two arguments. This is a merry-go-round without end. They will concede your point, then retract their concession, then fake left while whistling amazing grace. This is the usual response. They arent sure what to admit to so they try and run out the clock.
b) a creationist that will just throw up their hands and say, "sorry, dont understand alleles, lets talk about all the missing comets" (or whatever).
c) Ive read a few places where creationists are trying to use a process called homolgous recombination as some kind of way around the allele problem. Rather than allow the alleles came about from mutation they try and suggest that the new alleles are being constructed by the system itself and are thus guided. This is not your standard reponse though as these writers will usually lament the average creationists inability to see the seriousness of the allele problem for them. No evidence for this guiding but, hey, at least its an admission of a potential problem for them.