What should happen if a user is scrolling through items in a combobox with the keyboard and they press the Tab key? Should it behave like the Enter key, selecting the currently highlighted item and stepping to the next control? (I've seen this on webpages such as google applications) or should it behave like pressing escape to close the control and then jumping to the next control? (I've seen this in the built-in look and feel for on Java for the mac.) I'm biased myself because I often lose a selection I'm making when I hit tab by habit, but I can't find any accessibility guidelines that support using it to commit the value. I'll also note, it's a pain to implement in Java swing and seems to require an ugly hack.
If the answer depends on the platform where the application is running, then it may be a complicated answer. I'm working on a cross-platform desktop Java application (majority Mac, with substantial windows and linux populations) that we expect to eventually migrate to the web.