Undocumented feature? Hidden button between scroll wheel and home button?

[dvdmon]

I happened across this a few times by accident but didn't until today figure out what I was doing to get this to work. I also didn't see it mentioned in the manual, but I only looked through the manual, rather than reading it cover-to-cover.

Anyway, it appears that at least for me (using a Sprint HTC Touch Pro with orginal ROM), I can switch between my Email, SMS, and Picture Mail inboxes by clicking to the left of the home button and to the right of the scroll wheel.

In addition, it seems that this same button lets you cycle through the different tabs of the Touch Flo Interface when you are on one of its tabs.

This also brings up another point - I wonder if this "button" so to speak is programmable and if so how - what is it's equivelent name for programs such as AE Button Plus? And, do the other buttons on the the phone have this "extra" button - I experimented a bit but I couldn't figure out any extended functionality. Finally, does this "button" have any functionality in other applications or parts of the OS?