If it's compatibility with other UI guidelines, I think it's mistaken.
The reason that the user interface guidelines on both Windows and Mac (and virtually every GUI out there) recommend having TAB move from control to control and ENTER activate the selected button is for keyboard-only operation.
On the Mac, which is more mouse-oriented, normally only enterable text fields are selected by tabbing, but you *can* get all controls selected with a preference option.
SL makes the Mac's mouse-centric design look half-hearted. And it's kind of inherent to the design...
By default, TAB should only move from input box to input box in the same window. The main background window only has one input box... the chat bar.
Please consider reverting to the previous behaviour. At least make it an option, the way it is in Mac OS.