GPS is a good idea there. We also all carry GMRS radios. Particularly useful to us has been Garmin Rino's, which are GMRS radios with GPS that marks the location of each person when they transmit. Radio's alone are a little problematic because if you're off-trail, it can be very hard to describe to the others exactly where you are stuck.
West Yellowstone is a good mix of trails and off-trail play areas, hill climbing, etc. The grooming is usually fairly good - but it's useful to get a daily grooming report. Most of the hotels get that faxed from the Chamber of Commerce daily. You'll need to get a sled permit/stickers for both Idaho and Montana.
Riding the trail systems in Idaho and Montana around West Yellowstone has nothing to do with riding in the park. Yellowstone Park is worth seeing once. Not sure what the rules are this year (AFAIK, the NPS Winter Use Plan is still in effect), but as of 2007 (last year), to go into the park, you must rent a sled using BAT, and most have governers that prevent going faster than 40 mph (these sleds are dogs), which isn't a big deal since that's the speed limit in the park and every single ranger has a radar gun (it seems). You also can only have groups of 10 sleds and you have to go with a certified guide. Absolutely no off-trail riding in the parkPermits are required, and these need to be obtained in advance because they only allow 720 sleds per day in the park, and only a certain (smaller) number from each entrance. Most of the snowmobile rental places can provide you with guide, snowmobile and permits. It's expensive when you add up permits, sled rental, guide (and tip) about $300 for a day trip.
In the park, watch your speed, obey all stop signs scrupulously, single file ONLY, be careful where you park. Most of the park rangers there are dickheads and will write you an expensive ticket for any infraction. Outside the park, there are no sled restrictions, nor a need for guides. Speed limit is 50 mph and cops with radar guns on the 4 or 5 trails out of West Yellowstone are common within 4 or 5 miles of town, and most frequently seen in the mornings and evenings. Jet and clutch your sled for about 8000 ft.
I have several photos from various trips out there over the years
HERE and
HERE.