I really dont think fernando will be successful next year, not by a long shot. Lets look at the problems that he will face.
1) He will be with a new team and new engineers and mechanics which he will have to come to an understanding with, not just in testing but in the races when things start to (and they will) go wrong.
2) It will be the first year for McLaren on the bridgestone tyres and while the new control tyre will be new to all teams there will be others that have more contact with Bridgestone's basic philosophy which will give them at least a slight edge.
3) New and unproved car designer, This is the first post Newy year for Mclaren and while they have had to develop this years car without him it will be the first year that they have had to start from scratch. Another point is that depending on who his teammate actually turns out to be they are likely to get time to start deveoping the new car around themselves as Fernando cant drive the new Mclaren until at least the end of the season for contractual reasons. This brings us to number 4
4) The different philosophy of building cars. Each team has different ways that they like to build the car and it takes a while to get used to a different team's construction ethos. Whereas the Renault has been characterized by medium power with a very felixble and responsive chassis that is kind to the tyres but has a bit of understeer built in the McLaren for '07 will most likely be similar to this years car which seems quite different. This will take some getting used to (not saying he isn't up to it just that it will take time)
For those reasons not to mention the question mark over reliability which has been bullet proof at renault this year bar hungary, i think Fernando is about to come down to earth with a bump next year. Renault while not perfect over the past couple of years has always given Fernando an honest shot at the title. It will take time to make Alonso a consistent winner in the McLaren so it will be interesting to see when/if he is fighting for another title. There is the schumacher precedent with ferrari but there are also many for whom it has gone horribly wrong: Hill, Villeneuve and others who have left their championship winning team and never gone back. I hope for his sake that he does make it work but i dont think it will happen soon and definetly not next year.
PS. Sorry for the monster post