Show me a computer that has a dual core processor with optical audio (up to 7.1 DD or DTS) that is virtually silent and with such a tiny footprint, then we'll see who likes what. The Mini is in a class by itself.
AOpen miniPC (exactly the same size) and Dell Studio Hybrid (7.5 x 7.5 x 3.75" once you remove the outer shell, so not much bigger than the Mac mini). But the Studio Hybrid does have a Blu-Ray option and an HDMI port in addition to DVI.
The Mac mini hasn't been in a class by itself for a long time. And now with those EEEBox and similar very low-cost 'net computers, the Mac mini is also no longer the smallest consumer desktop computer either (specs aside).
Steve seems to love being able to say "we have the thinnest notebook", "we have the thinnest all-in-one desktop computer"... but competitors have gone and beat the Mac mini so many times, perhaps Apple has had to scrap their "Mac nano" designs a few times already, which would explain the lack of updates. Maybe they were waiting for a new part to become available (such as the 1.6 GHz dual core Intel Atom 330 perhaps, with some people saying that it makes a great low-cost hackintosh with the Intel D945GCLF2 mini-ITX motherboard).
I'm not saying that I want a very thin, Atom 330-powered Mac nano, but seeing the MacBook Air and the latest updates to the iMac, it's a safe bet that Apple will probably replace the Mac mini with a very tiny, very thin Mac nano. Maybe even with the footprint of the MacBook Air drive, so they can even exclude the optical drive from the Mac nano to make it even cheaper and thinner. The EEEBox and similar internet boxes also don't have optical drives and Apple has its "airdrive" thing that it made for the MacBook Air. People who still want an optical drive can buy the MacBook Air superdrive as a 99$ option and stack it on top of their Mac nano.
Me, I'd rather have a bigger Mac mini (same size as Time Capsule, taller wouldn't be a problem) with a 3.5" drive and a decent discrete GPU.