Although the Giants and Barry Bonds have supposedly agreed on a one year, $20 million deal, the deal still hasn't been finalized, and the San Jose Mercury News article I linked there has a bunch of vague quotes from Brian Sabean that don't shed much light on things.
The Mercury News does say that Bonds hasn't signed a "letter of agreement," so the financial terms of the agreement in principle that Bonds and the Giants supposedly reached last month aren't binding yet, and Bonds doesn't have to be put on the 40 man roster.
The article also says that some of Bonds' people think that the Giants are trying to find a way to get out of the deal, which would put Bonds back on the market...