It's not a paid celebrity endorsement (public servants are not supposed to have those) but, if he were able to take money for his endorsement, it would be extremely valuable to RIM, the company that makes Blackberries. Marketing experts have estimated the value of his endorsement as somewhere between 25 and 50 million dollars.

From the New York Times:
"“You always want the celebrity to be a good fit with your brand, and is anybody considered a better communicator right now than Barack Obama, or a better networker?” said Fran Kelly, the chief executive of the advertising agency Arnold Worldwide, who estimated that an endorsement by Mr. Obama would be worth $25 million. “It couldn’t have a better spokesperson.”

Mr. Shabelman put the value even higher, at $50 million or more, because the endorsement is worldwide.

“The worth to a company to have the president always talking about a BlackBerry and how it absolutely is a necessity to keep in touch with reality?” he said. “Think about how far the company has come if they’re able to say, ‘The president has to have this to keep in touch.’ "

As for why Obama may have to give up his Blackberry, you can see
http://www.mahalo.com/Obama_BlackBerry
The main concern is that the Presidential Records Act would force all of his emails to be archived on record, or even made public, so as long as you don't plan on being president any time soon there should still be hope for the rest of us Blackberry addicts.

I'd say no, it's not a celebrity endorsement, for twomain reasons.

One, he's not getting paid to use or promote the product.
Two, he's not come out and said much publically about the product. He lets his use of the product speak for itself, and others who are Blackberry obsessed are the ones who have pointed out that Obama uses it.