Yeah, I think any time there isn't an actual event in the karaoke room, it's open mic. Not that I know officially, but I'm pretty sure. >>
On that note, I have a question about the open mic: are we allowed to sing songs that aren't anime- or japanese-related? I know it's preferred since it is an anime convention, but since open mic is just for fun anyway, is it alright to bring in some other songs to sing? I know last year someone sang a song from Wicked, so I was just wondering. Oh, and the songs would probably have something to do with whoever I'm cosplaying, so it would be kind of related I guess. ^^;