I have posted a program that can log data from a serial port to an SD card.
This program can log data at 57,600 baud when used with a 328 Arduino and at 115,200 baud when used with a Mega Arduino.
The logger uses a new serial library that allows large RX buffers and has better error checking than the standard Arduino HardwareSerial driver. This allows data overrun errors to be detected so you can be sure data is not being lost.
The data logging program, the new SerialPort library, and a modified version of the SdFat SD library are posted as SerialLoggerBeta20120108.zip here http://code.google.com/p/beta-lib/downloads/list
I use a 1024 byte RX buffer on a 328 Arduino that provides 174 milliseconds of buffering at 57,600 baud and a 4096 byte RX buffer on the Mega that provides 347 milliseconds at 115,200 baud. A few SD cards may fail on 328 Arduinos since the SD spec allows occasional write times of 250 milliseconds. My experience is most SD cards can log sustained data at 57,600 baud on a 328 Arduino.