I've had the same problem. I'm using WP 2.3.2, Ruby 1.8.5 (the Ruby that comes with Leopard), TM 1.5.7 (1455), and whatever version of the blogging bundle that comes with TM 1.5.7.
The first time I upload a post--without a Date header line--the posting time as reflected in the blog is correct, but the posting time as reflected in the Date header line of the updated TM file is 6 hours earlier. (My blog is set with a UTC offset of -6.) If I edit the post without correcting the Date line, the blog will be 6 hours too early and the updated TM file will be 12 hours too early. Each subsequent repost will move the times back a further 6 hours.
The TM wiki has an entry about time problems with WP and the blogging bundle, but it talks about older versions of both WP and Ruby. My current workaround is to fix the Date header before reposting.