I bought a batch of Class10(up to 20mb/sec) SDHC Kingston 16G for $15 form MSY.
Worked great, but the latest batch failed with the mmc0 timeout problem. There are various workarounds involving recompiling kernels etc. but that doesn't seem all that userfriendly for the students.
There are a bunch of alternatives available:
Class10(up to 20mb/sec) SDHC Patriot LX 8G/16G/32G 8/13/24
Class10(up to 30mb/sec) SDHC SanDisk Ultra 16G/32G/64G 15/30/59
Class10(up to 35mb/sec) SDHC Patriot EP 16G/32G/64G 20/33/60
Class10(up to 45mb/sec) SDHC SanDisk Extreme 8G/16G/32G 15/21/36
I've got a handfull of these now:
Class6 SDHC SanDisk Ultra 16G for $13
Class10 SDHC SanDisk Ultra 16G for $15
So far RaspBMC has only worked with old 2G SD cards. Not sure that's even a problem. 2G is plenty for the software and even 16G is laughable for media (i.e. RaspBMC will need external storage anyway).