Are the DLLs produced by compiling XS code with MinGW compatible with a Perl built with MS VC++?

Assuming that the VC++ is VC6 then yes, if its VC7 then yes, but not if you use C++, if its VC8 then no.

The rule is that it doesnt matter what compiler was used, it matters what CRT is in use. Since stuff built with both VC6 and MINGW link against MSVCRT60.dll they are able to coexist peacefully. Similarly MSVCRT60 and MSVCRT70 are compatible in terms of C linking, but C++ exceptions will cause a problem. Luckily the use of C++ is rare in Perl.

Also as of later AS Perl builds you can compile using either MINGW or VC. If VC isnt available and MINGW is then EUMM will automatically use the latter.