Dell XPS 14......
Project Sputnik is a great idea that may be limited in its execution due to the older hardware involved. The XPS 13 was an awesome ultrabook when it was introduced and the idea of supplying a tuned/modified linux distro to work with the hardware was a good first step....but time marches on. Ultrabooks with larger, higher resolution screens, 8GB of RAM coupled with Ivy Bridge CPUs and Intel HD 4000 graphics are available at the same or lower prices that the beta XPS 13.
My ideal Sputnik laptop would be a Dell XPS 14 ultrabook -- it's got a 14" screen with a 1600x900 resolution, USB 3, BT4, GigE, an SD slot, and can be equipped with 8GB of RAM and an SSD with an Ivy Bridge i5/i7. Ubunutu running smoothly (linux already runs on Ivy Bridge/Intel HD 4000) on a new, well received and reviewed ultrabook would generate some buzz.
If Project Sputnik could could convince Dell to offer something like this to beta testers with a discount that would put at the same price as the XPS 13 currently offered that would be big, b/c right now you can get a Dell refurb XPS 13 for roughly the same price that a developer can get one now with a coupon. I know most volunteers didn't join the program to get a a super cheap ultrabook, but the reality is that while the Project Sputnik idea is great, the hardware involved is not competitive.
Perhaps a solution is to provide beta testers with an XPS 13 coupon so they can buy whichever XPS 13 fits there needs while identifying a more modern hardware platform.