Thanks Jason P
Update: TUAW reader Chris Forsythe wrote in reporting that he found Perian 1.0 buggy and that it's crashing on him. So far it's working fine for me but I have only tested it with Xvid and streaming MPEG-4 (the keynote). Let us know your results in the comments.
Update 2: readers are reporting that the file seems to have disappeared from MacUpdate. "I get to the Perian 1.0 page, but if you click the download link you get a file not found error."
Update 3: TUAW reader Yuvi writes that the MacUpdate link was not meant to be released.