Yeah, I figure it caps your # of powers at some point... but how many? Eh. Maybe 4 or 5 dailies and encounters, each, before looking at utility.
For what it's worth, what we 'know'
A 5th wizard has 2 encounter and 2 daily powers.
Wizards get
10 Utility
13 Encounter
15 Daily
16 Utility
Paladins get Encounters at 13 and 27
Warlords and Rogues get Dailies at 9
That's why I feel really good about 1-16 - in particular 21 on I'm fuzzy.
Given the rogue at-wills go from W to 2W at 21st, I suspect that you don't just replace them at 11/21. Now, retrain them whenever? Sure. I actually suspect that as you get higher level cool attack things you shift your lower levels into more specialized roles... like I personally think Fox's Cunning is lousy as your only encounter ability, but as your 5th? Sure.