I think Allan makes a good point -- mash-up as we've implicitly defined it is too broad a term to say much about the piece it describes. The qualities of the specific media being "mashed" tend to have more of an impact on the finished product than the fact that a "mashing" has occurred. By thinking of the "Yoshimi" album as a mash and looking at it in that context, we miss the basic fact that the album is music and should be looked at from a musical framework.
When we think of the music as music and the fake movie trailers as fake movie trailers, it's not as troublesome that the two media have about nothing in common. As a medium, music is about as unironic as they come. There's parody, I guess, and you could make someone laugh by looping a boy band over some rap beats, but that's not usually how it turns out. (although check out a personal favorite for an exception, and this one's pretty funny, it's a montage of a frustrated dude playing Halo) On the contrary, the music mashes are usually seamless, innovative, and pretty nice to listen to.
I think this has a lot to do with who's making the media. It's a lot easier to make your own soundtrack to a movie trailer than it is to mix a seamless track. I think Adam makes a really astute point when he says mashups are a way of subverting the one-way channel of the media, but I don't think that's what's going on with the music tracks. These guys are professional DJ's making professional quality products. They might be a bit different, but they're still a part of the one-way channel of music. The tonality and sonic quality of the tracks is so high because these tracks are meant to be enjoyed on an artistic level, not an ironic one.
Music lends itself to this mashup thing in a different way than images because manipulating songs is way harder than photoshop, and because the ear is a lot less forgiving than the eye. I'm not going to mind a poor photoshop job that makes a good joke, but there's no way I'm going to listen to more than a few seconds of a song with audio issues. Even with GarageBand and all that, intertwining tracks is out of reach for millions of people who use YouTube... and I have lost track of my point.
All I'm saying is the term's misleading because different mashed media can have a whole lot more differences than similarities.