A bit late to the party here, but my preference is for image #3.
I prefer the distinct border between the dark header & the light main forum, with the tab being the only thing popping up into the dark. The skin that uses all dark for the BG (#2) makes the tab look like an odd appendage to me. The skin that extends the dark header further down (#1), over emphasizes the curve on the lower portion of the tab and the corresponding rounded corner of the forum bit. Additionally, there is a bit more space between the search box and the user controls on #3. The others feel scrunched up to me.
This one gets my vote.