I don't know that forcing everyone into a P2Pool would really accomplish anything.  The large miners would not be hurt, they will simply continue mining their private nodes and carrying on like nothing has changed.  Pool operators will also not be hurt.  Shares will be found, payments will be made, fees will be taken.

The only real change by doing this, is that the mining operators (independent farms, pools, solo) get a smoother revenue stream.  Win for all.

Individual miners would have no incentive to organize themselves in to teams to support numerous nodes and large operators will have no incentive to break up their systems.  At best, (BIGPOOL) breaks up into 100 smaller nodes, but all 100 still controlled by the same people.

I really don't think that concentrated hashing power is as big a threat as most make it out to be.  Sure, someone might execute a double spend of their own funds, but the victim should have recourse.  Honestly, who's going to accept a $50,000 payment for a new car without requiring a few dozen confirmations before completing the deal?  It's a candybar?  Oh well, you just got shoplifted, call the cops.  Virtual service?  Well, you should have required payment in advance and waited for a few confirmations.

I wonder though, if there isn't a real threat of collusion between pool operators to impose new rules on the entire community.  For example, the US Dollar is controlled by 7 people (currently 5).  They can, without explicit approval of the government, print all the money they want.

This is where Bitcoin is headed if we only have a small handful of pool operators controlling the global hash power.  What if those few decided to get together and issue a code update that allows them to generate all the BTC they wanted.  Sure, if it happened tomorrow, everyone would just walk away.  What about when Bitcoin is the only currency in wide spread use?  We'll have to launch yet another form of currency to compete and hopefully take down the Fiat Bitcoin.

No, I'm not worried about double spending.  I'm worried about the will of a few people being imposed on everyone else.