Obvious, but handy. It would be nice to be able to read the iTMS explicit tags with applescript and write a flag to the comments...

I use this trick as well, and identify my 'clean' playlists by putting the copyright symbol in front of the playlist name [since it looks like a (c)]. I wish that we could access the Explicit tag through iTunes, as dfbills noted, and that you could turn Explicit on and off as a Setting on the iPod. I could erase half of my playlists that way.

I do something similar. First of all, I surround my tags by [] to make sure they're unique. Here are some of the tags I use...

[Cover]
[Explicit]
[Bootleg]
[Live]
[Concert]

etc...