You named 'em all that I know of unfortunately...
It's not very hard at all, you just have to decide how you want to do it. The idea is basically to copy the `while(next_album()):` loop into album.php, which is where the subalbums of that album will be printed. But of course, it's entirely up to you how they're displayed and styled, so that's the hard part.
All the themes released with 1.1 in a month or two will be ported to support sub-albums in some way, just in case you're wondering.
If anyone is interested in adding sub-album functionality to the current themes, please let me know!