For casual-party DJ use, don't forget that iTunes has built-in crossfade capability.
It doesn't handle beat-synchronization or other mixing effects, but with a 12-second song overlap, at least you don't have to do a manual cross-fade of distinct sources.
If you're taking requests, all you have to do is slip the requested song into the right position in the playlist while another song is playing (try not to do this *during* the cross-fade).
Since I was a participant in the party at which I used this system (not just a DJ), I also took advantage of the Keyspan Digital Media Remote. This allowed me to pause the music from a distance during an announcement, and it was quite useful when I decided to discreetly skip a song that wouldn't have matched the tone of the evening as it was developing (bad track set-up, perhaps -- but using a random playlist worked out fairly well otherwise).
Plus, having an unattended TiBook sitting on the DJ table, with the silver back and glowing apple logo facing the crowd, is just cool. And hey, style counts in my book.