Go get it. If you take the introduction dates of all the Mini's (the major updates), and average the intervals, you will see the next one comes around november 7th. Pretty far away. The don't buy is really weird on this website.
So there might be a marginal speed bump sooner, but for Ivy Bridge, you have to wait till the holiday season at least. But if you know the Ivy comes at the end of the year, a speed bumped Sandy won't come any more.
Dates: Interval (days)
11-01-05
28-02-06 413
07-08-07 525
03-03-09 574
15-06-10 469
20-07-11 400
average is 476 days
Ivy bridge is coming around 7-nov-2012
And the Mini 2011 will hold value as it is available with discrete graphics (the Ivy one will come with HD4000 probably), you can pump in a SSD and 16Gb, allows serious expansion and external drive bays over TB, and because it runs software from 2001-20xx as it will be the last Mini to handle Rosetta.
Another thing: Ivy Bridge is a Tick in Intel terms: same architecture, smaller fab process. So don't expect big speed bumps from it. It will run cooler mostly, and maybe Apple will use it to make the mini even more mini (iFixit repairability <3, SSD and RAM pre-mount, HD room removed to shrink the case) instead of using the fab process to pump a quad i7 into the base model and thus cannibalizing MacPro's/iMacs/MBP's.