I've thought a little about this, too.
Many of us do a sort of quasi-mastering on our two buss when printing reference mixes. Obviously when sending the tracks to a mastering engineer, you pull that two buss limiter off, and maybe lower the overall level a bit.
I think if people want to squash their mix, that's probably ok. If you make a note in your post that your mix is unmastered, or has two buss limiting, etc, it'll give people a better idea of what they're listening for.
But for now, there's no hard guideline.
What do you think about having a 'mastering' section, where someone could post a 24bit wave stereo recording, and other could post their mastered versions of the song? A little off topic for here, but might be fun!