Sadly, the Raspberry Pi has a bootloader bug that makes it incompatible with some cards.
Somewhat off topic, but just FWIW, the R-Pi has been reported to show about 20 MB/sec both reading and writing to a "SanDisk Extreme SDHC 4GB class 10" according to RPi Performance - eLinux.org
Somewhere around June/July 2012 there were some bug-fixes to the SD card I/O routine firmware in the R-Pi, which improved compatibility with several different types of cards. Most cards perform slower than the above example.