I talked to one of the rep’s from Cervelo Customer Services about this. The new improved brake will be retro fit-table for all P4 models and this also counts for the improved seatpost bolt.
Some other improvements, like special bottle holders which can be used in the existing aero bottle location of the P4 ‘might’ not be retro fit-table on all versions of the P4, because of changes in the 2011 P4 Down Tube.
More info in the last months of 2010. I’ve heard that Cervelo will be servicing existing P4 owners with a warranty upgrade kit for some of the improvements, like the rear brake. They say because they are great full for the P4 owner feedback they’ve received and won’t let down existing P4 users that currently have rear brake ‘issues’.
I think this is hopeful.
Eric
From your lips to God’s (or Gerard’s 
ears…
My “fix” for my P4 rear brake was: take everything apart, totally bugger the plastic (delrin?) bushings for the rocker arm thingie, so I remove and discard them.
Lube all moving parts w/ good grease. Build back up (much cursing involved, as it took several tries to get everything in just so), and hey! Both brake arms actually work now! Whoda thunk it?
Yes, the rocker arm doo-dad isn’t snug like it was, but more’s the better, as it moves freely now, and hence both brakes move freely now too.