This plugin adds a little tab beneath the admin bar in WP 3.1 onwards which on click shows/hides the admin bar.
Change Log
02/03/2010 Apologies to those who downloaded it yesterday (01/03/2010). Just tested it out from the repository and realised it didnt work because the folder name that WordPress provided was different to the folder name that I was building it in so a link was incorrect. The path has been corrected now.