Only Apple knows for sure, but historically, Apple does not release new product after late November; they typically go into their peak holiday selling season without changing their product line.
Allegedly, Apple plans on shipping the new Mac Pro sometime in December, but that is not a consumer product and its sales aren't driven by the holidays.
It is highly unlikely that Apple will release a new Mac mini before the end of the year.
I have a strong suspicion that Apple is working on a new industrial design for the Mac mini, a taller one with a smaller footprint (like the AirPort Extreme but in aluminum or maybe something like the new Mac Pro). If Apple just wanted to upgrade the internals with the same mini pizza box chassis, they could have done so months ago. Of course, this is pure speculation on my part, without a shred of rumor/evidence for me to work off of.
Anyhow, my two cents...