Hi Leighton,
We have started a project to unify all our WinForms products into a large bundle (would still sell individually too) similar to our WPF setup. Once piece of that is to move them all to .NET 3.5 SP1, which will work fine in .NET 4.0 (as does the existing .NET 2.0 compiles... they work fine in .NET 4.0 too).
We don't support Client Profile though because our assemblies currently have dependencies on System.Design, which we hope to eventually get rid of by moving that data out to separate design assemblies.
But again, .NET 4.0 is fully supported right now, just not .NET 4.0 Client Profile.