I have basically the same setup - 2.6GHz MacBook Pro (1440x900) and an external 1680x1050 display. I found the same problem - until I switched the monitor arrangement so that the external display became the primary Mac OS X display (when connected). In the Display Preferences >> Arrangement, drag the menu bar indicator onto the external display. With build 4.0.3810 in coherence mode, the "Windows" display still appears as one, large monitor, but now it will be the height of the external display. You need to fiddle with window positioning so that you don't have your secondary windows cut off on the laptop display (adjust the display arrangement) to line up at the top, bottom or whichever way works best for you). The big difference with .3810, though, is that now when you maximize one of the Windows windows, it only maximizes on one of the displays, instead of spanning both - a very big improvement. This is still not the ideal of having Windows see two, distinct monitors, but at least it is progress!
I, like the rest of us here, eagerly await the full, multi-display support!
2.6GHz MacBook Pro/200GB 7200 rpm/4GB RAM/OS X 10.5.6/Win XP Pro SP3 virtual machine