I'm seriously considering buying the 2.3GHz quad-core Intel Core i7, but after reading this should I be patient and wait to buy the next model?
If I can't scrape together enough money for a new Mac Pro then I'm going to be in the same situation and will definitely wait if I can.
The question really is what you need; an update to Haswell CPUs won't increase CPU performance by a huge amount, though it has a few neat improvements for things like virtualisation performance if you need that.
The big benefit of Haswell will be the updated graphics, as all the possibilities are a big step up from what the current Mac Minis have. I'm hoping for Iris graphics across the line, maybe with Iris Pro on the high-end model (maybe models if they keep the server option). Though I'm not sure how likely Iris Pro is; the MacBook Pros can have it, but Mac Minis aren't really positioned as a pro product so who knows for sure?
In my case I'll be replacing an 8-core early 2008 Mac Pro, and the current quad-core Mac Minis already outperform me on most CPU tasks now. With the updated integrated graphics they'll be competing with my GeForce 8800 GT GPU too, so while it won't be a huge upgrade overall, it won't cost me that much more than repairs will. I'm still hoping to get a new Mac Pro, but if not, the mini is what I'll have to get, and I wouldn't want to get a current one for fear that an update is only a few months away at most, may even be a lot sooner.