I have tried a few dictionary programs on my Nokia N800, but I just came across one that may be more generally useful. StarDict has a Linux and Windows version:
http://stardict.sourceforge.net/index.php. It has also been ported to the Nokia N800:
http://maemo.org/downloads/product/stardict/.
I haven't loaded up the Nokia version yet, but a quick test of the Windows version looks promising. StarDict seems to work in most programs, popping up a window, with a definition of the word that is being pointed to or highlighted. Here is a screenshot of StarDict in FBReader. For some reason, in FBReader you have to point to the word that is two lines above the one you want. This bug doesn't occur in other programs that I have tried.
As there is already a Windows and Linux version, and this program is GPL, perhaps some of the E Ink reader vendors should look at porting this to their readers.
BTW, if you don't like word popups displaying all the time, go to Options and check "Only scan while modifier key is being pressed". Then you have to press "shift" to get the popup.