I'm not sure we have to wait all that much longer...
They said it can be end of March/early April, I would assume they would try and release it around then, or at least that beta you speak of. I don't think Sony wants to adopt SE's update reputation, so I would hope they would release it around when they said they will release it.
Jelly Bean is in the works, sure, but it's all down to rumors at the moment to when it will be actually released. If we are one update behind for awhile, I don't really care as long as it's optimized for the phone, and not some half-baked, rushed OS port.