well, it's hard to say. It was upgraded with the imacs when the latter went to core 2 duo, so that option was available for the mini, but apple passed on it.
So there's an argument that the mini will only be upgraded again when the imac is - I mean, if apple wanted a c2d mini available at the same time as these c2d imacs, they could have done it when the imac got the c2d.
On the other side of that argument is the fact that the mini is currently the only 32 bit system left, which it was not at the time of the last update, and the fact that it's a pretty bad deal compared to every other machine apple sells.
My own opinion is that the mini will see an update when intel's next-gen integrated GPU is ready and not before. The last update was nothing but a processor bump, and I think they will not do the next update until they can add both new graphics and a c2d.
Have you considered an imac? They are much better deals than minis right now.