I bought a new one for one of my employees last week.
Depends what you want it for. I have a new and a last gen mini, and they're really comparable, I don't think it's as big a "future proof" issue as it is with, say, a tower or iMac. Plus the current have Thunderbolt, are 64-bit, and have SATA6, so I doubt that other than a gpu upgrade and faster bus, there won't be a HUGE difference in the next model (which may come out next week or next year, who the heck knows?)...
And Liquinn is totally right. Buy what you need when you need it. There isn't a model of mac out there right now that I would call obsolete, other than the towers (which are still great machines, just that you can build a hackintosh that's twice as powerful for half the price because Apple isn't keeping up the pro line well enough [even more true when you consider that that statement was pretty much true on the release date of the current tower model as well]). Speaking as a designer, the pro line seems like it's currently only concerned with volume costumers and the support they need, not staying current. For anything other than a server farm or a desperate need for PCI cards, I'd go with an iMac over a tower right now.
/unrelated rant