Things are going great with the beta.
We've been slowly inviting developers to try it out to over the last month and a half. Just didn't feel it was stable enough a few weeks ago to let everyone in at once then have to help dozens of people with the same issues over and over.
That said we're looking pretty good now and should soon invite all Sony developers to try it. We've had a few games using it demo'ed at various events and you should see quite a few of them at E3 in a few weeks.