Looks like those of us running SharePoint may be stuck on .NET 3.5 SP1 for a while longer but that's no excuse not to have .NET 4.0 installed side-by-side for use by other apps. One of the guys on the team, for instance, recently updated the custom crawler we run and needed pull in some of the Entity Framework features only made available in the 4.0 release; of course Microsoft will tell you the various .NET runtimes can be installed to run side-by-side and we managed to get away with a lot during the whole 2.0/3.0/3.5 onion thing.
Of course this all gets interesting very quickly when you've got a console application built against 4.0 that references assemblies (i.e. the SharePoint assemblies) built against version 2.0. Naturally everything worked fine in dev but as soon as we hit UAT, this exception cropped up:
An error has occured: Mixed mode assembly is built against version 'v2.0.50727' of the runtime and cannot be loaded in the 4.0 runtime without additional configuration information. [sic]
To address this, a simple .config element was added:
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0" />
Notably, the useLegacyV2RuntimeActivationPolicy="true" attribute was required and we found this out the hard way by not having in place initially (dodgy internet code most likely!).