A statement has been put out by AT&T lets all customers know that Windows Phone 7 devices require a "certified high-speed microSD card" in order to work optimally. Specifically the Samsung Focus right now is expandable with microSD cards is warned about in the AT&T statement, them then noting the fact that "Certified for Windows Phone 7" is not yet marked on any microSD cards on the market. Updates came into Engadget after this new initially broke letting us know that microSD card compatibility with Windows Phone 7 devices isn't just based on speed class - other factors including the number of random read/write operations play a role, too.
More on the subject found by Engadget included documentation from Samsung including the fact that once a microSD card is inserted into a WP7 device, it "will no longer be readable or writable on any other devices such as computers, cameras, printers, and so on." This includes formatting. Scary! Furthermore, they were told that it's not so much to do with total throughput, as Microsoft would imply, but mostly to do with the random access speed. Sadly, the type of cards these WP7 will be good to work with are currently only manufactured in bulk (which explains why the cards that come standard in the phones work just fine.)