Just in the process of ordering one of these magicshine rear lights from dealextreme to complement the front, with a splitter cable to run both off a single battery pack. Only worry is that it might be too bright...(£37 shipped, but can be bought from MagicshineUK for £45 (+£7 p&p) to get it quicker.)
3W rear light, jesus. I want one. my 0.5W RSP Astrum is already painfully bright.
the road behind you will look like this: