On our cruise in Feb 04, Celebrity had started the option of adding these to your shipboard account, but you had to have this completed by the day before the last day of the cruise and could only do for the standard amounts. We waited too long and had to come up with the cash anyway. (You can only write one check for $200 on board, but they will make change for you)
Some people at our table did this and they were given a small card to hand the maitre-d, the waiter, etc., stating they had used this option, so it didn't look like they were cheapskates not tipping on the last night when its customary to give the staff tips.
This is more convenient than trying to get together exactly the right amount of cash to put in the tip envelopes, so we'll be sure to get it together early enough to use this option on our next cruise.