Also most of the managed languages that need to be compiled actually do this whilst you are writing it. This is what makes C# compilations so blindingly fast.
C# compilation is still very fast without this. I use an embedded C# compiler in my game engine to build cs files on launch and I can build a whole directory full of dozens of cs files in less than 20 seconds.
(OT) XCode 4 builds C++ code while you're writing it, and it's really annoying, especially since they decided this feature was so great that there was no need to be able to initiate a compile of an individual source file in a project any more :/ I had to buy a new top of the line Mac Pro just so I wouldn't have to wait 10 minutes every time I tweaked a template method to make sure it wasn't going to break my build.