[TippeX]

"Fx 3.5 continues with the additive approach that we introduced with Fx 3.0 while still not introducing a new CLR. As a reminder, Fx 3.0 simply added to Fx 2.0 without changing or removing anything in the existing assemblies (it added WPF, WCF, WF and CardSpace). So, Fx 3.5 simply adds some assemblies to the existing set of DLLs that Fx 2.0/3.0 comprises of. This means that simply installing the framework should not affect your existing solutions. Note however that Fx 3.5 depends on Fx 2.0 SP1 and Fx 3.0 SP1, which will ship at the same time."

"The .NET Framework 3.5 is shipping later this year at the same time as Visual Studio 2008. The .NET Framework 3.5 is an additive release, meaning that it adds functionality in terms of classes in assemblies to the previous versions 3.0 and 2.0 of the .NET Framework. Since .NET Framework 3.5 doesn't replace (or install side by side) with .NET Framework 2.0 / 3.0 it is able to retain backward compatibility for applications built to .NET Framework 2.0 / 3.0. However there are two required service packs before you can install .NET Framework 3.5. They are .NET 2.0 SP1 and .NET 3.0 SP1, fortunately they are included in the distribution of .NET Framework 3.5. They will also be available for separate download from Microsoft and on Microsoft Update for existing customers who do not need .NET Framework 3.5 but still want the fixes."

guess its not backward compatible without those components...

apparently it emits IL code for .net 2.0 and backwards... lots of confusion about it, but then its newish, so expected