Good question.
We counted Harrison Ford a short term client when he was in town shooting K-19. Our staff found him quite amiable.
Director Guy Norris rented a couple of iMacs for he and his kids while shooting in Toronto.
Sylvia Tyson wandered in one night with her Mac guru in tow to pick up her first computer.
Seinfeld apparently had a TAM on his show always.
Talking with a consultant in the US he said he got snagged by I believe Deniro for a permanent "on staff" position for his Mac. Nice job eh.
The Hermitage staff in Moscow use a Macdoc Mac to catalogue their art - some of which is being shown in Toronto right now.
Those are all that come to mind from personal experience but Kawasiki's page had lots of famous users listed.
I'm surprised about the Presidents given the White House mind set about "non-standard" stuff.