I can't speak to Best Buy in specific but it does seem as if the 30 gig white units are the ones showing up at most stores around the nation. Some places have black ones and some have 60 gig units too so all the variations are shipping. I think that the scarcity of the 60 gig black and, to a lesser extent the white is due to the fact that large chunks of the original shipments are being sold to people who reserved iPods before they hit the shelves. At least, IMO.
Best strategy: look at a lot of places and look frequently. Don't believe them when you call them and they say they don't have them. Go to the actual store and look.