If we manage to get 55% or better on Feb 1, then for the next two week's I'll be sending out the message "Upgrade or you might be on the short end of a blockchain split come Feb 15" -- and I expect the result to be a large majority of miners supporting P2SH by the Feb 15'th switchover date. If we're still at 55% on Feb 7'th then I'll be worried, too, and might advise miners to push the hard switchover date a couple of weeks (if they're using the patches I'm creating then it is a command-line argument to bitcoind).
The real danger is for the 45% -- after Feb 15 (assuming the switchover happens) all it takes is for one old miner who is including non-standard transactions in their blocks to create a block containing a transaction that is invalid under the new rules. They announce the block, and any miners who haven't upgraded would happily build on it, only to waste some hashing power because the 55% majority will sooner or later reject their chain.
However, I don't think anybody will accidentally mine a block spending an 'invalid under the new rules transaction' in it (the number of people mining non-standard transactions seems to be very small), and it seems unlikely an attacker would waste time solving one or more blocks that they knew were going to be rejected by a majority of the network.