i just checked my used belt w/ 800 miles on a 06 600sdi, against my new belt.
it showed 30 thousands of wear on it.
how much wear is acceptable before having to replace it?

I don't have my manual in front of me.......but I believe it states that minimum width of the belt should be around 35mm

I have 2800 miles on my belt. I didn't measure it, but it looks brand new still. I put my new one on anyway this fall just so it doens't dry rot before I ever use it. Now my "get me home" belt is used instead of new.

Your owner's manual will list the new width and minimum width. I simply run them until I can't get the belt deflection set right using the adjusters on the secondary.

If you don't have the owner's manual, you can download it from www.ski-doo.com, under sleds, the operator's guide.