The other, real reason to stay away from the Dell Mini 10 is that it has the Atom Z520 processor, which, like all Z-series, uses the GMA500 chipset, codenamed Poulsbo. Sure, you can buy the Mini 10 with Ubuntu already installed, but it's Ubuntu 8.04 (Hardy) with a video driver that isn't ported to Ubuntu 9.04 (Jaunty). Blueprints and bugs indicate that there's no movement forward on this.
What does that mean? It means that lots of folks have bought the 10v with the intention of re-installing Ubuntu (maybe UNR) and getting 9.04. Unfortunately, they're left trying to import old packages and make them work. Hours are wasted.
Instead, you can get the Mini 10v, which ships with the N270, a chip which will drain your battery much faster than the Z520. It also comes with the ancient and extremely well supported GMA950 chipset (also a power hog). Installation, though, will be a breeze, and everything will work out of the box.
"V for Victory" I say!
Update: Well, it turns out that there's a new PPA open. Add the following to your sources:
deb http://ppa.launchpad.net/ubuntu-mobile/ppa/ubuntu jaunty main
deb-src http://ppa.launchpad.net/ubuntu-mobile/ppa/ubuntu jaunty main
apt-get install xserver-xorg-video-psb