I recently checked out the most recent alpha of Haiku and it was great. Very fast and full of potential however I couldn't help but notice that it looks precisely the same as the old BeOS and while this isn't a bad thing I'd hoped for more small details and improvements (many of which I am sure will come on their own). In the interest however of contributing what I have I did a mock-up of some minor improvements to the basic tab shape, which I feel bring the classic BeOS tabbed look into the present.
the mock-up is here: http://img688.imageshack.us/i/haikuos.png/
as you can see the upper left hand corner is rounded resulting in a friendlier feel, while the right side of the tab has been made diagonal for a sense of motion.
I hope you like it. :)