Technically speaking, if they wanted to, they could relabel much faster processors as slower ones, underclock them and have them stay cool.
Hard drive manufacturers used to (still do?) "destroke" a hard drive. They could keep a huge pile of 80 GB drives on hand, and turn them into 60's, 40's and 20's if they needed to with a firmware burn. It sounds crappy, but when you sell to corporate America, some of them have imaging tools that can't handle larger drive capacities.
I wouldn't be totally surprised if Apple were to go get some more recent chips, clock them down a little to maintain cooling, and pop them in.
I'll be buying a Mini2009 when Snow Leopard goes live, so I'd love a CPU bump.