Well I wrote (a command-line) program that was about 2000 lines long in C++ and during most of the development period I used g++ with the -ansi & -pedantic options.

Later I was able to get my program to compile on Visual C++ with absolutely no modification.

Ah, that's different, that's just compilation (and yes, I've seen compilation incompatibilities too; VS usually requires code changes to compile).

I was referring to the binary interface. What I'd want to do (and generally can with C) is compile a library with any compiler, and link it with any other code, with no recompiling.

You see this all the time with Unix, say; maybe Sun compiles a bunch of stuff with their C compiler, but you can easily link to what's in /usr/lib using GCC. But with C++, it's typical for the loader to not link C++ libraries that were built differently; and if you're really unlucky, the loader won't notice and instead you'll see subtle binary errors at runtime.