to answer two questions at once, the way i hear it, the Megas will be available for sale in-between the regular series. so between series II and III would be Beaker, between III and IV will be (apparantly) Animal, etc. that way there is a constant Muppet presence and people don't lose interest...and they're not having to spend all their money at once (people are more likely to buy when they don't have to choose between gettin 2-3 smaller ones, or one big one).
at least, i THINK that's what's going on...if i'm wrong, i'm sure one of the guys'll let us know.
to sum up...my guess is to expect Beaker to be available for Christmas