Entirely different setup I've used, generally just to keep my ass unlost when I go hiking/photo taking in the woods, but it could be used for MilSim if you were playing on a big enough chunk of real estate:
I use a Pocket PC with a bluetooth GPS receiver, and a piece of software called GPS Tuner. (actual setup is an iPAQ HX2410 running WinMobile2003 and HOLUX GPSlim 236 receiver, with GPSTuner 5.0)
Why this setup? GPS Tuner is designed for offroad navigation, unlike most other software (TomTom, iNAV, etc) which are designed for road navigation. More importantly it allows you to import and calibrate maps of your choosing, including from Google Earth. You can also scan a [paper] topo map and use that, or even a hand drawn map if you can put coordinates to locations on it.
Actually the feature list probably offers a better glimpse of its capabilities:
http://www.gpstuner.com/overview.htm
One interesting-looking feature I haven't gotten to use yet is position sharing with another user (so you can see where they are on your map, and vice versa). This could be interesting for coordinating two fire teams, I suppose.
The HOLUX 236 receiver is small (about the size of a Zippo) and doesn't use much power (I've used it for 10 consecutive hours, and it was still running).
If I power manage my pocket pc properly I can get about 10 hours of runtime with the bluetooth radio going.