I'm having this same issue with WP 2.9.2 and Postie 1.3.4. I tried upgrading to 1.4 and 1.4.1 but with either of those, I couldn't get Postie to post anything - all configuration tests succeeded, but for whatever reason Postie wouldn't retrieve the messages that I could see on the server.
In any event, after reverting to 1.3.4 I've gotten Postie working again except that email sent from my mobile phone that have photos attached get "Scheduled" for a week in the future.. really weird! And I am unable to get the offset value to change - I edit it, click save, but upon page refreh the same offset displays (-7). However if I look in the wp_postie_config table it appears to have save the correct value. Really weird, and unfortunately this makes Postie pretty unusable for me as the whole point of using the plugin (for me) is to be able to post photos in realtime from my mobile.
I've confirmed my mobile phone is sending the correct date. Text only messages sent from the phone get posted immediately instead of being scheduled, as do email messages sent to Postie. I'm at a loss as to why a message with a photo attachment would trigger delayed posting, but there it is.
Thanks for any help,
Steve