Just as a followup, I got the SD/MCC adapter board from www.gravitech.us and used it with a Transcend 16GB SDHC Class 10 card. With this I was able to log over 4 hours of data at 7 kB/sec with no dropped characters, which is good enough for now. I used the SdFat "AnalogLogger" example with minor modifications (mostly just increased the data written to the card, write one line of data every 6.2 msec, and call logfile.flush() every second.)
With the basic 2GB Sandisk microSD cards I had been using, I could not go even one minute at 3.5 kB/sec without some dropped characters.
#define FLUSH_INTERVAL 1000 // msec between SD file flush
...
logfile << buf; // log data
if ((m - lastflush) > FLUSH_INTERVAL) {
logfile.flush(); // flush data to SD
lastflush = m;
cout << buf; // send data to serial port
}