I think the deal revolves more around
distribution rights than a total buy-out. Although recently
John Riccitiello admitted faults with EA's developer acquisition strategies. It's hard to say where EA will take a new developer in light of such acknowledgements, but I believe Valve's gushing praise was more just a way to entice EA to work on a market that Valve doesn't much want to be involved in: Physical distribution.
Let's face it, digital distribution is the future and Steam has matured into a pretty damn robust platform. Not only that, but I'd wager that profits from electronically delivered games are much higher than physical and with less headaches to boot. Valve could be withdrawing from a logistical headache and bringing EA on board to ease the burden with their well-established shipping channels. Also, since digital delivery of full retail releases to consoles is a while off yet, they're likely looking to tap into the massive console market with a partner who has the resources and experience to release games in a much larger volume.
This is all just conjecture of course, but as a small, proud independent developer for 12 years, I can't imagine that a highly profitable developer / publisher would want to sell up right now. If anything, they're wriggling the worm to get an idea of their market value, so they can work on bigger things.