We have two configs at work:
primary client [browser / client] in mini-itx case, mounted on the wall
512MB DDR RAM
notebook 20GB HD
SuSE 9.0 / Windows XP Home
1GB DDR RAM
2 80 GB Seagate 7200 RPM in RAID [mirror]
2 NIC [Heartbeat / HA]
Jakarta Tomcat 5.0.19
The main difference in speed is with the hard drives. The notebook hds tend to be very very slow. And I would not use them for DVD playback or multimedia purposes unless you want to manually install / struggle with the appropriate drivers. You can do a search on the issues on viaarena.com to find out more on which mobo supports which distro / release.
As for my own opinions -- I've used VIA mini-itx mobos ever since its inception. I use one at home as a jukebox / carry-case storage box. It works fine in this regard but let me reiterate that the main issue is with the video drivers.
As for mutimedia small form factor boxes, there are several commecial products available. You can find some reviews on http://www.mini-itx.com
hope this helps,