I don't see Apple skipping two releases just to put Broadwell in the Mini. Especially when you consider that Broadwell won't be around until late this year. I think it is more likely that they were waiting for the Mac Pro to be released.
Now that's out of the way, and new Haswell mobile processors were announced two weeks ago. There were several that would work for the Mini. The i7-4860HQ would be a perfect BTO for it.
My guess is that Apple will be refreshing the rMBP and Mini at the same time with the Haswell refresh in the very near future.