As i've said in another thread, this isn't a bad deal NOW. But you have to consider the following [considering you are signing a 3 year contract]:
1. What will 500MB/1GB cost you in 2008-2009
2. What broadband tech will be prevalent in 2008-2009 [Wimax/cheaper forms of ADSL etc]
3. What laptop will be required to run things like Vista in 2008-2009 .
I would consider a 24 month contract, but 36 is pushing it a bit in all 3 areas mentioned above. Just look what happened in the last 2 years in all 3 points mentioned. Laptops didn't change as much, but we didn't see a new OS in the last 3 years either.