Work has already been done by Gisle Aas of ActiveState to allow XS modules to be compiled with either the MinGW gcc or Visual C++, regardless of the compiler that Win32 Perl was built with (see Perl change #27555 for details). I'm pretty sure that Perl has been able to be built with either dmake or nmake with Visual C++ for some time now. It would be nice, however, and I'm sure you would agree, to allow users their choice of compilers on whatever operating system they choose. I would hate to see this project be a step backwards in Perl support on Win32.
With Windows Vista, all indications are that support for the free nmake will go away. The nmakes provided with recent Visual Studio's or .NET SDKs will still work. In exchange for losing free nmake support, however, we will have a usable and programmable shell that may be able to be used to provide a sane Configure script. So, I see Vista as a great new opportunity and challenge for porting Perl rather than an obstacle.
As far as working with commercial compilers, Perl is everywhere and should be able to be compiled with any sane C compiler, commercial or not, just as it is supported on almost every free and commercial operating systems. Whether the C compiler is commercial or not should be irrelevant. Support for Visual C++ 8.0 will be provided. Its simply a matter of time. I consider it to be a major bug that it isn't supported at this time. This is why I see this support as important for this project.