I don't think it's highly unlikely at all. It's been a few years since the mini as we know it was released, and the last update of any kind was quite some time ago. It's due for some tweaking in form-factor/design.
I think they will keep the same footprint, but go taller for the next model - at least on the high end. They may keep a small, lower priced model. Perhaps take the current high end, drop the price to $499, perhaps with some small changes.
A new model with a C2D, 3.5" HDD, more ports, improved video, maybe some better built in speakers would be nice. I figure they would only need to move about 2-2.5" up to accommodate all that; about the height of the mini plus the miniStack HDD enclosure available now. That makes room for the HDD and some additional cooling space and frees up the volume that the 2.5" HDD occupied for better (stereo) speakers, and some more cooling space.
Not sure if the C2D/X3000 run much hotter than the CD/GMA in the mini now, but a 3.5" HDD would add some heat, and I'd rather they used the space for additional air movement, passive radiators, etc than add more/louder fans to a new mini.