Once you go for lambdas, lazy computations and other functional techniques in programming, you may want your code to be mostly immutable. With ReSharper 4 we didn't do much about data analysis, like do not detect immutable objects or methods. However, one analysis is already available to you: highlight mutable local variables. Open Tools menu, select Options, browse for Fonts and Colors under Environment group. In the Display items find ReSharper Mutable Local Variable and change the appearance as you like. I use Bold font. ReSharper will now highlight every variable that changes its value after the value has been already used.
Try it, and tell us what do you think!