I don't believe they have plateaued. Look at what's being released tomorrow: the LifeDrive....it's a brand new type of PDA. There can always be more storage, more ways to take advantage of wireless technology, ability to control more objects, etc.,etc..
I really think that there is a ways to go as far as PDA technology is concerned. I'm waiting for the day when I can start and warm up my car with my PDA from inside my home!