I wonder if anyone else has played around with Linux running on other ARM based boards... The Raspberry Pi is certainly the cheapest, and that in itself is compelling, but it's also the only one intended to run Linux (rather than android, which is of course mostly Linux anyway).. But, being the cheapest, it's also the lowest spec... 700Mhz ARMv6 with 256Megs RAM is absolutely huge for an embedded computer, and as an educational tool... but it's not going to be your next desktop!
So, I've played with the MK802, which is a 1Ghz ARMv7 processor, with 1Gig RAM... it's smaller than the Pi, but it has no GPIO, and no Ethernet, or Audio out, or Composite Video.... it was quite convincing running Android, and Linux too... but without GPIO you would be using up your USB and paying more for some external interfaces. It was also twice the price of a Pi, but if you want to turn your old HDMI TV into a smart TV with Android, it's pretty good... it even has built in WiFi... but if you want to learn about Linux and perhaps control some external project then I'd stick with the Pi
The star of the show is more expensive still, but with 4 cores running at 1.4Ghz, the ODROID-X is an absolute rocket ship compared to the Raspberry Pi... It still only has 1Gig of ram, which is a bit of a limitation for a desktop, but it is certainly the fastest by far, arguably faster than even a dual core Intel Atom.... It even has GPIO, although I've yet to find any documentation for that.... but as yet there is no 3d support, and the sound driver is still a work in progress...
Very pretty tho:- http://www.fsck.co.uk/temp/ODROID_Screen3.png
I think the conclusion is that the Raspberry Pi is probably the best supported hardware, and the most practical for electronics projects & education, certainly the most affordable... and I suppose thats what matters.... although it is worth noting that binaries compiled on the Raspberry Pi (ARMv6) will work on ARMv7 devices... although both devices need to be ARMHF (or not)... so if you are building .deb packages on the Pi, they will probably work on a whole range of ARM platforms beyond the Pi, including Ubuntu's ARM port.
Finally, I know some people were bitter about Ubuntu not being supported on the Pi, you're not missing anything... because only the quad core ODROID could run the Ubuntu desktop properly, and without that, there's not a whole lot of difference between Ubuntu and Debian. If there is the odd package in Ubuntu that isn't in Raspbian, then as long as it doesn't depend on lots of other packages then it's easy enough to build it for ARMHF/Raspbian... even I could do that fairly easily now!