Gubbi - it seems McLaren is not able to fullfill his wishes on huge salary (due to an increase in costs, as they will need to pay for the Mercedes engines in the future) and there are rumours that Vodaphone may be decreasing their sponsorship. From what I've read, McLaren are also reluctant to give him more leeway on private sponsoring - they have been traditionally very strict (for reasons I can't quite figure out).
This may be all due to contract negotiations, but to me it looks McLaren can't afford (or doesn't want to) to meet salary demands and are even suggesting a pay cut - and pushing the angle that they are a top team and the only team that is able to offer Lewis a probable race-winning seat.
At the end of the day, what I find more worring than any talk about money or being able to keep the real trophies is that he does not seem to get on well with the team anymore. I've had this feeling ever since Whitmarsh got Jenson into the team and I think there may be more to it than what is known in public domain.
There was an article in German "Spiegel Magazine" on this topic specifically and there were a few interesting comments. One of them specifically that the scandal regarding Hamiltons tweeting of telemetry data caused quite a stir within the team - and apparently, quite a few within the team are "fuming". This may be intepretation on behalf of the german magazine, but if there is any truth to it, it to me sounds like there are deeper problems within the relationship of Lewis and his team.
Anyway, no facts here nor insider knowlege, just a gut feeling...
EDIT: Just wanted to add (and felt it would be more appropriate to add it as an edit than another post), that while Hamiltons ambitions are clear - I think the issue may be that he can't share his ambitions with his team. Perhaps in other worlds - a higher pay (or more sucess) is not always (more) rewarding if there's something wrong in the working environment.
James Allen has something written up
Spiegel.de on Hamilton and why he may move to Mercedes