The bad news is that only 52.7% of the users has .NET Framework 3.5 installed on their PCs, but as you can see from the table they become more and more very fast:

  Mar-08 Jun-09 Aug-09 Oct-09
Dot Nothing 28.12% 25.60% 25.53% 21.40%
.NET 1.0 2.59% 0.60% 0.40% 0.30%
.NET 1.1 23.22% 13.00% 8.41% 7.70%
.NET 2.0 27.41% 23.20% 23.02% 11.60%
.NET 3.0 17.67% 14.40% 10.01% 6.30%
.NET 3.5 0.99% 21.82% 32.63% 52.70%