I Just did it.
UPDATE: Just returned from Canada on Saturday with my new sled. You have to check in with Customs on your way into Canada and declare the check that you are bringing in if more than $10,000 Canadian. Just 5 minutes paperwork. On the way home, you need to check in and register that you are importing the sled with US customs. You may need some EPA documentation showing the sled meets US EPA requirements and will need to physically show them the VIN number to make sure it matches the paperwork. You'll also need a bill of sale. But it took me 10 minutes on the way home. Very easy on both sides of the border.
Original: I also found a great deal on a sled that I could not pass up.
I was going crazy from all of the different web sites and advice I was getting, but it was actually very easy.
At first I was told I needed to register for a SCAC code and fill out all this paperwork, pay fees, etc. Then I finally got to some people who actually knew the deal. I found it's best to talk to the border agents directly at the sites where you wlil be crossing - be sure and get their names when you do. Mine was a private party sale so there is actually very little to it. You simply go pick up the sled and at the border when you reenter the US you show them a bill of sale. You have to go in and fill out some paperwork. My sled was originally built in Minnesota, so they don't worry about the EPA compliance and there is no duty. It was very simple. The only down side is of this deal is that my sled only has 3 km on it (about a mile) and 3 years left of a 4 year warranty, but the warranty becomes void as soon as I bring it to the US. I am still trying to get Cat to give me some assurances that they will cover me if anything goes wrong when I take it out next year. If anyone has experience with that or how to actually get them to respond to emails and letters that I sent them, I would appreciate it.