Not that I know of. Note that if you do add a variable, it will get removed by the compiler in release builds anyway…
Update:
This functionality has been added to VS2013.
You can see the return values in the autos windows or use $ReturnValue in the watch/immediate window.
The value can only be seen directly after returning from the function, thus the easiest way to access it is by putting a breakpoint on the function call and step over (F10) the call.
Update for VS2015: boo! unfortunately, it doesn’t appear to be in VS2015 (devenv v14)
Update for VS2017: it’s back. (devenv v15)