It seems to me that one reason PS2 hasn't died faster, is that a game can be developed in parallel, one for PS3/X360 and one for PS2/Wii or just for the PS2/Wii (ie, casual mini-games, etc)

So, it is still worth developing the game mainly for the 120M+ PS2 crowd and then porting it over to Wii to get another shot at 20M+ users there.

However, I have to believe that the Wii is a lot more powerful than the much older PS2.  As such, are there / will there be games that, even ignoring motion sensing of the Wii remote, that can be done on the Wii, but are too much for the PS2 to handle?   

third party games? Yes... (please don't port Chicken shoot)
first party games? no way in hell they will port it.

so... I rather they don't frankly.

There are games already: Resident Evil 4 Wii Edition is a direct port of the Gamecube version which had to be downgraded to work on the Playstation 2.

Games like Super Mario Galaxy, Metroid Prime 3 and Super Smash Bros. Brawl are also graphically too advanced to work on a Playstation 2.

Remember, we are talking about a console that is roughly 2 times as powerful as a Gamecube, which was more powerful than the Playstation 2.

The Wii can handle everything the PS2 can give, so devs always port from PS2==>Wii, not the other way around (usually)...

well....of course 1st party games will never be ported. 3rd parties may - depending on the game (because of the motion sensing)

it depends if the game is primarly made on PS2 or on the Wii.
Wii ==> PS2 port is harder to do since the Wii is more powerfull than the PS2
But u can also made a Wii==>PS2 port by adapting the GFX to the PS2.
I suppose it is not so difficult.

If you are talking about the pure technical aspect, see: Super Mario Galaxy and Super Smash Bros. Brawl.

 I was thinking that SSBB (or Sony equivalent) would be, since there is so much going on at the 30 fps.  But just was wondering if any PS2 fans would say that the specs were such that PS2 could handle all of that game at its frame rate.

Games that ran at 30fps on the PS2 ran at 60fps on the Gamecube with improved textures and effects, games that ran at 30fps on the Gamecube run at 60fps on the Wii with improved textures and effects ...

This means that you can port a PS2 game to the Wii, or do cross platform development, but you can not port a Wii game to the PS2 without a noticeable downgrade ...

Without any downgrading at all, a game like Brawl would sink to sub-5fps speeds. Models are too detailed and there are too much textures + effects for the PS2 to handle them.