I think that yes, the trend will be that agencies will bump their prices to generate more revenue. This will become increasingly significant when the industry has matured and there is less room to grow by attracting more customers. Another reason is that the funky players that offer nothing to photographers/designers are being dumped and the 'cream' and best managed corporations remain.
I don't know how immediate it will be and whether these impacts will speed it up, but you can definitely look in that direction.
One last interesting feeling is that we are going to end up seeing regular $4 to $5 sales instead of 50 cents. My thought is because the sites know that designers can pay for this especially since they once paid $500 - so 99% off the previous price paid is pretty awesome still for them, although I'm really hoping that something good comes out of this and photographers begin to get a larger cut.
-gareri