I suppose when someone at Stardock on a 2560x1600 monitor figures out what the deal is on the Diamond skin and lets me know what the problem is I will make adjustments. Until then I reckon the only answer is to not maximize.
I am on a 24' 1920x1200 monitor and cannot duplicate it.
You said a 'couple of other themes' did this... how many have you tried..? I think you'll find its much more than a couple if you check many more.
I have a feeling its going to be easier to fix the issue within WB than to change thousands of skins that I assume will have similar issues.
Knowing that 30' 2560 monitors arent that widespread yet, it may be some time before there is full support for them