Create a task for targeting multiple .NET framework versions about fake HOT 3 CLOSED

I don't think this is enough... How do you know from the FAKE script which dependencies are 4.0 and need to be replaced by equivalent 3.5 ones? IMHO the only way to do this is "manually" (or at least this part), i.e. the developer has to mark the necessary references as 4.0-only and also add the replacements to be used for the 3.5 build (or Silverlight build, or Mono build, etc).
The best solution for multitargeting I've seen is Castle's (see Windsor.csproj), it just uses multiple MSBuild configurations, each with its own output path and constants defined for ifdeffing, and references with conditions to exclude/include them. This also has the benefit that you can easily choose the configuration (therefore target) right from Visual Studio. So in this case, all the pieces are already there... no need for any specific FAKE task, just set the desired Configuration, or if you want iterate through a list of available Configurations.

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Mauricio's suggestion works for me. I could use a course on setting up configurations. :)

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Just reread this and on second thought, it should be possible to iterate through the project references, fetch the framework version from their assembly metadata (or project target in the case of a project reference) and XML-poke as you said, using some sort of convention for the location of the equivalent assemblies (e.g. NuGet's convention of NET20, NET35, NET40, etc).
Though there are probably some pitfalls with this... and I still think handling this at the msbuild level is better.

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