I just committed a minimal implementation of "accounts", as discussed a few weeks ago in this thread.
If you're using the command-line or JSON APIs, you should be aware of a change that might make your code break: the sendtoaddress method will return a hexadecimal transaction id (256-bit hash) instead of the string 'sent'.
All of the 'label' commands have been renamed; the old names are still supported but are deprecated and may eventually be removed.
If you're developing a web service using bitcoin, the new 'sendfrom' and 'move' methods can make it much easier to keep track of customer account balances. The API is intended to be used like this:
Create a new account: just generate a unique account ID in your application (maybe customer's login name).
Get a bitcoin receiving address associated with the account:
getaccountaddress <account_id>
Note: multiple bitcoin addresses can be associated with the account
Send bitcoins from the customer's account:
sendfrom <fromaccount> <tobitcoinaddress> <amount> [minconf=1] [comment] [comment-to]
Will fail if <fromaccount> doesn't have enough bitcoins (otherwise returns transaction id)
Move bitcoins between accounts:
move <fromaccount> <toaccount> <amount> [minconf=1] [comment]
Will fail if <fromaccount> doesn't have enough bitcoins
Report account balance:
getbalance [account] [minconf=1]
The empty-string account is a little bit special. Any coins received on bitcoin addresses not associated with other accounts is credited to it, and coins sent via the (old) sendtoaddress method are debited from it.
Coming soon, I hope, will be a gettransaction <txid> method to return details of a transaction that is stored in your wallet (like which account it was to or from and whether or not transaction fees were paid). And listtransactions, to get an accountant-friendly itemized list of all the transactions that went into any particular account balance.