I've downloaded the NUnit 2.5 source and opened the VS2008 solution in the VS2010 beta. Once the conversion finished I opened all the projects and changed the target framework setting for all the projects to ".NET Framework 4.0". I then built the solution without any errors. I can now use the NUnit GUI app to run tests built for .NET 4.0. I've not done exhaustive testing of this build so there may be problems, but for my purposes it works fine.

Update: It is not necessary to rebuild NUnit. I discovered that if you add the following to the relevant NUnit config file you can run a test dll built for .NET 4.0.

Under <configuration> add:

<startup>  <requiredRuntime version="v4.0.20506" /></startup>

and under <runtime> add:

<loadFromRemoteSources enabled="true" />