I've been interested in ReadyBoost for a while now but it's hard to find out which devices will work and which will not.  Performance specs do not provide enough information to tell because often they list speeds in only bursty situations.  Some manufacturers will tell you on the packaging but most don't mention it.  For those, the only way to tell has been to buy the part and try it out.  Even though you might be able to return an incompatible unit, that gets old fast.  I just ran across a cool site which, as of this writing, lists the compatibility of 1109 devices.  Of those, 674 are supposed to be compatible.  You can search by manufacturer or just peruse the list of compatible devices.  Quite a valuable resource.

Note:  I'm not on the ReadyBoost team and have no connection with them.  This is in no way an endorsement of this list or the devices on it by Microsoft.