April Reagan's lastest post (http://blogs.msdn.com/aprilr ) is full of great information on the C/C++ compilers that ship with the PSDK, .NET SDK, XBox and with Visual Studio.  She explains why you see different versions from time to time and what those different versions mean.

An interesting thing she talks about is how the Windows OS is an internal customer of the compilers and a laboratory for improvements and innovation in the compilers.  It's yet another example of how we "dogfood" our own software before we feel it's good enough to ship to you.