I've had a report from a bitcoin trading site that upgrading to master caused problems with their current setup.
The 0.8 behavior of walletpassphrase is:
> walletpassphrase 1000
walletunlocktime = now + 1000
> walletpassphrase 10
Error: Wallet is already unlocked (old unlock time stays)
The current behavior of walletpassphrase is:
> walletpassphrase 1000
walletunlocktime = now + 1000
> walletpassphrase 10
walletunlocktime = now + 10 (overriding the old unlock time)
This is likely just a matter of documenting it in the release notes and RPC documentation so that developers can update their code before upgrading.