From what I understood before it was to preserve compatibility with Mac OS X Tiger's Aqua, which I can understand. However, since Transmission now requires Mac OS X Leopard or higher that reason pretty much lost its meaning.
I realize it's a detail.

PS
Really nice to see you got rid of those odd looking buttons in the bottom bar, just having the glyphs makes for a much cleaner appearance.
PPS
Is there any word if a shifting animation such as iChat and Adium will be implemented when transfers change position? I'm guessing using Core Animation that shouldn't be much of a problem anymore, compared to the Pather/Tiger days.