Here's my opinion (not based on anything but what pops into my head): The PS3 and the 360's architecture are very different. The 360 is a classic setup, developers know what it can do and how to push it. Games will get better on the 360 in terms of visuals and AI and CPU crunching tasks. But the 360 will hit a barrier to its performance sooner than the PS3 will reach its max.
With the PS3, developers will have to take a different approach to designing games. I dont think your going to see PS3 games blowing 360 games out of the water graphically but what the PS3 should be able to do is handle games that require a lot going on at once, games of greater scale.
Games that require lots of AI or lots of processing should shine on the PS3. But the problem is that these types of games will take time to develop, they will come but it may be a while. And at the end of the day the type of games that suck up huge amounts of CPU via AI and processing tasks arent always the best selling games or the most popular.
At the end of the day graphics sell games and I cant see the PS3 blowing the 360 out of the water graphically for a good few years. What you might see is PS3 games having more happening on screen without slow down as apposed to the 360 which has to scale back to make the game playable.
And my final point is can developers justify putting all this extra effort into maxing out the PS3? If games are only using 30% of the PS3 at the moment and looking this good is it worth throwing time and money at a PS3 game to get upto 40% when 30% will do? Sales of PS3 arent high enough to warrant huge investments yet. Developers are here to make money and the PS3 just wont make enough money for developers at the moment so whether the console gets pushed towards max isnt a done deal.