You aren’t the only one with no success.
My server is running OSX 10.4 PPC, apache 1.3, php5, mysql 4. Have tried the htaccess approach but it didn’t help out. Installing WP 2.5 on MacIntel running Apache 2, php5, mysql 5 and the media uploader does work properly. Something on server side me thinks.
I basically decided to stick with the no-flash-uploader plugin until a solution shows up (or I get a new MacIntel as a server).