If Apple intend to keep the Mac Mini (and I suspect they do) then chances are good they're waiting for Broadwell, as it has much better power/heat efficiency and comes in smaller sizes, but it's not due to later this year.
The question at that point is then whether the Mac Mini will get the update, or if it will be made to wait until other machines have had Broadwell upgrades first.
Personally I'm hoping that Apple has had the new Mac Mini design (even if it's only a minor change) ready for a while, and is just waiting on the processors and will do a release as soon as possible. The only thing is that there are other machines that Apple will want to upgrade to Broadwell too, but I'm hoping they'll just do a bigger release than normal to catch up.
It's not really like the Mac Mini is a big threat to the other lines; new Mac buyers that don't have existing monitors will still be more interested in iMacs if they can afford them. Even if the Mac Mini were to end up ahead of the iMac in CPU/integrated graphics performance, only those who are more knowledgeable would buy one over the iMac, plus there's no reason Apple couldn't update the iMac lineup at the same time.
The biggest problem really is that laptops seem like they could get priority for the Christmas season, which might relegate iMacs and Mac Minis to updates early next year instead. I just hope that's not the case as I'd still like to get an upgraded Mac Mini, but am getting increasingly tired of waiting
