I think we have two different worlds: the 645D and the 'rest'. Of course, in a few years the 645 will get a successor, but as far as I can predict not in 2011/2012.
For the 'other' DSLR, I think this range is a likely possibility:
I - unknown - Pro, Full frame - I don't know when
II - K-7 - Prosumer - will be replaced in 2011? Each replacement is of course also an upgrade!
III - K5? - Enthousiast - introduced in 2010? based on K7/K-x
IV - K-x - Entry level - will be replaced in 2011?
More than this is not needed: K10/K20D, K200D and K-m can be discontinued (and some are already
)
Lower as K-x is in my opinion not the market for PENTAX