This is not a hardware error in my opinion it is software related so if you can not find the culprit i would start uninstalling what you have installed from the last backwards untill the "fault" clears up, failing that i would re-flash back to stock and then you will know for sure it is hardware or software related.
There is no "electrical" or physical connection between the enter button on the N900 keypad and the off button.
He already uninstalled everything he installed within two weeks of this problem starting or so, according to the very first post.
ALSO: there's no connection between shift, fn, and other keys, yet I regularly get (well, when I don't have R&D mode enabled, which currently is never anymore specifically because it keeps me from having to deal with spontaneous reboots/shutdowns) spastic reboots from my N900 when I use certain key combinations. (Typically for me it's shift+fn+arrow key - I have fn+arrows mapped to Home, End, Page Up, and Page Down keys - +shift helps with text selection when I write my typically long rant posts tremendously.)
You don't need an electrical or physical connection between buttons - you just need something to go wrong to have the device go "**** it, emergency shutdown time".
@OP: I had a similar problem with spontaneous reboots correlating to kb activity as you can see from the above, although it was never as reliably linked to one key.
You can try enabling R&D mode and disabling the watchdogs and lifeguard reset - for me, that has significantly reduced "spontaneous reboots" - I presume something crashes in the background, and gets revived just fine by itself when the lifeguard reset and/or watchdogs don't get the chance to kill everything in a mindless panicking shutdown.
You can also test to see what happens if you, say, switch backspace and enter keys, to begin with. Then see if the hardware enter key causes reboots when backspacing, or if the hardware backspace key causes reboots when entering. From that you'll know if remapping the keys will actually help at all.
