I'm not sure where you're getting your info, but my two year old Kingston 256mb USB2 memory stick is 100% ReadyBoost compatible. So is another of my ancient original (non-hi-speed) Sandisk 256mb SD cards.
My next removable flash storage purchase will be one of those new "convertable" units -- it's an SD card that also houses a USB connector too. Link for reference:
http://www.supermediastore.com. (I am not affiliated with that site, it's the first hit that came up in google)
Update edit: I went and captured a screenshot of my old-assed Kingston traveler 256mb card being used for readyboost by the event manager.
Click here to view at 1450x1050 resolution