Kind of on par with recent upgrades
I think the mac mini upgrades are similar in kind to the powermac upgrades from last year. Basically, the processor change is the biggest thing and they added a few bells and whistles (more usb ports, spdif, front row/remote) and kept the price point pretty much the same. The low end one is VERY similar to the "upgraded" mac minis that were being shipped out from late last year. I bought my sister the high end one and aside from the processor type (intel vs. ppc) & bells & whistles, it's pretty much the same for the same price.
Old one: 1.5ghz ppc g4, 8x DL superdrive, 80gig HD, 64MB graphics card, bluetooth 2.0 (retail price $699 - I got it for $620. $645-$25 rebate from amazon)
New one (upgraded): 1.5ghz intel core solo, combo->8x DL superdrive (+$50), 60gig->80gig HD (+$50), 64MB integrated intel graphics card, bluetooth 2.0 (retail price $599 + $100 in upgrades = $699) + bells & whistles.