I submitted a Connect item about this (here), but as you can see it won't be making it into this release.
To tide myself over I wrote a version that is built on anonymous types, Reflection.Emit, and Funcs. You can download it here. This is a proof of concept...no warranties...don't use in production...blah blah blah.
As a sample, here's how to create an instance of IEqualityComparer that thinks strings are equal if they are the same length:
Equals = Make.Func(
(string x, string y) => x.Length == y.Length)
I had to use Eric Lippert's trick to get lambdas and inferencing working in anonymous types.