Modern IDEs already support the static version of what you want. You can ask Eclipse for all the call sites of a method, for example. This will not perform a text based grep, but a semantic code search. A dynamic version of this that works on a concrete execution trace instead of the static code base also exists. You choose a point in time on a timeline and ask "who called the currently active method" and it will give you the exact call site that called the method at that point instead of all possible call sites. For example researchers at CMU have created the Whyline. You can watch a video of it here: http://www.cs.cmu.edu/~NatProg/movies/whyline-java-demo-web.... Also related is a technique called slicing.

The Roslyn CTP for Visual Studio is an API for the C#/VB compilers. Visual Studio itself uses this for e.g. refactorings. You can write VS plugins that examine the abstract syntax trees of the files in the project, instead of working with the flat text.