[pyre]

From http://design.canonical.com/2010/07/the-ubuntu-font/:

  > Today all Ubuntu Members will have access to a
  > private PPA so that they can use, test and enjoy
  > the font.
  > 
  > The font is scheduled for completion 8th August
  > and this is when we aim to open the beta to everyone.

So the they are opening usage to a select group prior to the actual completion of the font's design.

[rdtsc]

I think that makes sense. Perhaps the foundry asked for it. They wouldn't want to any negative comments because the work is not finished yet.

[Groxx]

And they probably (rightfully so) fear a wave of mostly-uninformed (read: non-typographic-geek) comments.