I bought a Samsung 32Gb Class 10 SDHC card for my Bionic and I'm getting great speeds. After tweaking the cache size, the speeds are even better. I ran some tests and here are the results I got with different cache sizes:
128k: 9.29Mb/sec read, 9.21Mb/sec write
(default)
512k: 9.73Mb/sec read, 15.42Mb/sec write
1024k: 8.94Mb/sec read, 15.22Mb/sec write
2048k: 9.48Mb/sec read, 15.34Mb/sec write
4096k: 11.16Mb/sec read, 15.84Mb/sec write
(BEST)
8192k: 9.51Mb/sec read, 12.91Mb/sec write
Note that even with the default cache size of 128k, speeds are very close to advertised for a Class 10 card -- so it's hard to see how any limitations could be in place with the Bionic. I'm running it with the cache set to 4096k on boot and even large video files now load instantaneously, which is great because I enjoy streaming video off the phone to my TV via the HDMI dock.
For those interested, I used
SD Booster to tweak the cache size and
SD Card Tester to run the speed tests. I tried every SD card testing app in the Market and none of the others would allow me to change the path to /sdcard-ext, which is critical obviously or you'll just end up testing the internal memory. It's tricky even with this app, but if you keep tapping in the top-right corner eventually it gives you the option to change the path. If others want to try it I'd be interested to see what kind of speeds people are getting with various SD cards on the Bionic.