I wouldn't blame him if he did quit, but I don't think he should.
He needs to show loyalty to the rest of the team who have helped him get where he is now. It's up to him at the end of the day really isn't it.
The only problem, if he did quit, would be where next? I can't see a team kicking a driver out to make way for him, so if he is to race anything he's looking at GP2 or F2 I'm afraid. Would be fun to see him in the BTCC lol. Getting his arse handed to him by Rob Collard 
Matt Neal would have a rival in terms of unfair driving 
Nothing could match that lol. I know it's off topic, but when He barged Turkington out of the way, it was 'a subtle, veteran passing move', and when Shedden (I think) did the same to someone else, it 'wasn't very clever or too subtle' according to Tim Harvey. Double standards exist in Touring Cars too apparently lol.
I think Lewis should stay in F1, and you never know, if he finishes the year at McLaren, he could have repaired his relationship with the team, or if he still wants to leave, there's bound to be an opening somewhere. In the end, how much effort he, and the team puts in to their relationship will determine where he will be this year, and in '10.