It is a file produced by the ResolveAssemblyReference build target. MSDN has this to say about it:
Visual Studio attempts to execute
targets with certain names when it
loads a project. These targets include
Compile, ResolveAssemblyReferences,
ResolveCOMReferences,
GetFrameworkPaths, and
CopyRunEnvironmentFiles. Visual Studio
runs these targets so that the
compiler can be initialized to provide
IntelliSense, the debugger can be
initialized, and references displayed
in Solution Explorer can be resolved.
If these targets are not present, the
project will load and build correctly
but the design-time experience in
Visual Studio will not be fully
functional.
If I interpret this correctly, I’d say that the file is used to help the IDE provide proper IntelliSense and assembly reference status in the References node. It is a fairly expensive operation since there are potentially a lot of assemblies that can be referenced. So instead of doing this repeatedly, the .cache file can help make this quick. Deleting it isn’t an issue, it will be recreated when the project is reloaded.