Like the other fellow said, wait until WWDC is over. Typically, I say buy it if you want it/don't wait, but with WWDC and several updates and introductions pending, it only makes sense to hold out a few days.
Regarding the processor, the argument that Apple doesn't want the Mac Mini to trump the 13" MBP is quite sound. In the past, I can't recall them ever releasing a consumer-level product that bested its pro line. They do often have consumer setups that are on par in some ways with the lower end Pro rigs (like the current Macbook 2.4 vs. MBP 2.4), and as such I'd say the most likely update to the Mini's CPU would be to offer the variants currently available in the 13" MBP (2.4 + 2.66). That said, I read a blog that stated a 90% chance of Mac Mini i5 + 320M at WWDC, but where that person was drawing their information from is anyone's guess.
Sticking with Core 2 Duo chips would jive with a 320 M update, as the i-line of processors require Intel's integrated graphics to be on board...and to the best of my knowledge, if Apple can't cram integrated plus nvidia in the 13" MBP, they probably can't do it in the Mac Mini.
So, rambling out of the way, this is my prediction for the Mac Mini: A WWDC update, bringing the specs to the following -- 2.4GHz C2D + 2GB Ram + 250 GB HDD + 320M = Low End. 2.66GHz C2D + 4GB Ram + 250 GB HDD + 320M = High End. HDMI, blu-ray, and all the other talked-about stuff is unlikely, unless Apple pulls a fast one and really gives the Mini a proper refresh.
