eleuthria is right.
The bad-for-not-upgraded-miners scenario is:
+ Somebody gets a valid BIP16 transaction into the chain.
+ They then spend that transaction by broadcasting a 'standard', non-BIP16 transaction, BUT they create a signature for the BIP16 part that old miners consider valid, new miners consider invalid.
+ The old miners will add that 'poisonous' transaction to their memory pool and if they include it in the blocks they mine then those blocks will be rejected by most of the network.
makomk pointed out a while ago that I hadn't noticed that old miners will accept spends of half-valid BIP16 transactions, because the old code doesn't check to see if a transaction is spending a non-standard input. He's right; one of the changes in the 0.6 release is an AreInputsStandard() method, to make upgrading smoother next time.
Executive summary: if you are a p2pool or solo miner you should upgrade before the switchover date (April 1, if all goes well) or there is a good chance you'll produce nothing but orphan blocks. I welcome suggestions on how to effectively get that message out to the community.