if you just want to log data (and not necessarily read it) i have an sdlogging library that saves to fat16 formatted SD cards. i use it to log GPS & sensor data: GPS datalogging shield for Arduino
Is anybody using this library? It overcomes some of the limitations of the libraries everyone is discussing here.
I came across this one too, which supports fat32 and sdhc: Google Code Archive - Long-term storage for Google Code Project Hosting.