I don't know why that bug would exist... the PM send code uses the same header redirect that the rest of the board uses. Nothing should be sent via POST when the inbox is loaded.
I'll do some debugging to see what's up...
Edit: I can't reproduce it in Opera... I have an idea what's causing it though, and I consider it to be a caching-related bug in IE and maybe Mozilla. Let me know if the problem still occurs now.
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