I see it as a nice little mini server for my home. I plan to get one put my iTunes to use with my wireless Airport Express to pipe music around the home. I also plan to use hook up my external HD and have it process my DVDs and ReplayTV files that I create. I'm a Powerbook owner. I edit video on my Powerbook but hate having to watch it compress DVDs, ect. It will be a mini file server/downloader. I'll store all that stuff I want to keep, but don't really need on my Powerbook. I may even setup a VPN so I can get at it on the road. heck you can get a nice $79 CRT and most of us have extra mice and keyboards around.
In the future I may hook up a El Gato EyeTV to supplement my ReplayTV.
The Mac Mini is the first step to an Apple Set-top/Home Network Mac.