If you have a windows machine and can solder a 4-way header onto your standard-format arduino (diecimila, duemilanova etc) then you can reprogram the bootloader:
http://www.geocities.jp/arduino_diecimila/bootloader/bitbang_w_ide_en.html
When I last tried with a mac (July 09) it wasn't possible using the mac's FTDI driver. Instead I used Windows running under parallels.
See also this post/thread: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1258854659/7#7