According to comments linked from DaringFireball, the opinion is that Jobs had to release the shares to pay taxes on the 10 milion shares of Apple he was issued:
More or less, Jobs just had 10 million shares vest, and he sold just enough of them to the pay the taxes on all of them. If he hadnt sold any, hed have had to pay about $300 million in taxes out of his pocket.
Appleinsider posts a similar report:
In order to meet his tax obligations on the 10M restricted shares, which vested this month, Jobs elected to net-share settle -- essentially allowing Apple to withhold and pay to authorities the portion of the 10M shares that would meet his tax payment requirements.
According to AppleInsider, Apple will retire these shares and will have no effect on Apple's current stock price.