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I was just there last week and looked around for something, all in vain. In the end I went to Santa Monica (I was staying near there anyway) and went to the AT&T store there.
Also -- if you've got an unlocked iPhone, and you want data access, make sure you get the 100mb package and 'enable' it by visiting Unlockit - APN Changer for your iPhone and adding the profile to the iPhone. Otherwise it won't work. (Obviously the AT&T people won't tell you about this, either, given they disabled data on pre-paid SIMs back in June. Took me a good half a day to figure it all out.)
L.