<p>I'm looking for a library or source code that provides guard methods such as checking for null arguments. Obviously this is rather simple to build, but I'm wondering if there are any out there for .NET already. A basic Google search didn't reveal much.</p>

Given Microsoft's
Code Contracts
coming out with .NET 4.0 I'd try to find one which is mostly compatible, if possible - and if not, write it yourself. That way when you upgrade to .NET 4.0 (eventually) the migration will be easier.