I believe it was Ubuntu also, but I will confirm this and amend this reply later today if it wasn't.
Update: I can confirm GeoHot did indeed use Ubuntu, to quote from his blog:
The issue is, we don't know if it will work yet until he tries it... the last thing we'd want to do is start posting various hardware attempts and have people running out ordering parts only to find out that none of them actually worked.
CJPC should be able to post the Linux details (download link, compiled kernel module, etc) beforehand though.
I wouldn't worry about that... if you are in the middle of things and get stuck, just post where you are at and they will be able to help you once someone here (CJPC, most likely) goes through the process successfully.