My OCD gene has really gone into overtime on this, so I thought I'd come back here and share what I've learned.
For LAME encoded files with gapless information, and for it's own aac files, iTunes uses the gapless info from the file itself. (There is some debate on what it does with Nero's gapless info). For older rips, whether it be fastenc, Gogo, or whatever, iTunes does a "psuedo-gapless" analysis on the files.
Gapless inforormation is stored in this ITUNSMPB tag ONLY for it's own rips (aac and mp3). When it does the gapless analysis on foreign rips it just keeps the info in it's "iTunes Library.itl" file.
Exporting your library does not export the gapless info, sadly. If you export your library, delete the iTunes database files, and then re-import, you can see this. iTunes does the gapless analysis all over again.
There also doesn't appear to be a way to get this info using the Apple SDK. Apparently they are a bit secretive about this. They probably don't want people importing their old rips into iTunes, then doing some tagging/scripting voodoo, and getting this info into another application. Damn them!
So... even if MC ended up supporting the ITUNSMPB tag, it would only apply to iTunes rips, where the info is in a file tag.
There are so many things I dislike about iTunes, but I will give credit where credit is due. They took on the challenge of gapless playback and they did a pretty good job. I loaded my iPod with a bunch of gapless albums (all older, "foreign" rips), and they playback pretty much seamlessly regardless of the codec.
The idea of doing a "gapless analysis" and then storing that data, either in the database, or in the file, is not a bad one.
You can't blame them for cooking up their own gapless tags really, considering they are doing gapless with old files without LAME headers, aac files, etc...