My understanding is that Premium is the default. That Starter and Basic are OEM only - since they are aimed at Netbooks and the low end, respectively and will therefore be available only with the cheapo box they were originally installed on. And that Ultimate is supposed to be a rather limited edition in terms of distribution - The removal of the 'Ultimate Extras' should kill most of the desire to have it for the few people who haven't figured out that MSFT won't actually deliver on that with Vista.
So for most, the choice is Premium for home users and Business for Small to Medium Business users. 32 bit or 64 bit.