As far as I understand Infopop's position (I'm sure they'll advise us if I'm mistaken).
UBB5 will continue to be available in the members area for a while after UBB6's release. While it is available, it will continue to be supported. However, I doubt you'll see anything but security patches.
I can say with near certainty you'll never see version 5.50 (and probably not 5.49 either).
And personally, I doubt Infopop will release any rights on UBB5 - just my opinion, but I see no reason for them to. They still own the copyright, and want to encourage people to move to UBB6.
Anyway, I'll be pointing this thread out to Navaho for him to give a once-over.
Graeme
PS See below - that's about as final an answer as you're going to get.
