It certainly is going to be strange to see him in the McLaren overalls. Actually, I was expecting him to quit Formula 1, and to continue his career in the WRC (where he can compete with his Fiat Punto S2000, because the S2000 rules apply for the WRC in 2010). I'm really looking forward to the new season. With Alonso at Ferrari, Raikkonen back to McLaren, possibly Kubica at Renault (Alonso's replacement), and the possible Barrichello/Rosberg switch (Rosberg from Williams to Brawn, and vice versa for Rubens) it certainly is going to be an interesting season.
However, these facts also bring up some questions. First of all, can McLaren handle two top-notch drivers? It didn't work out with Alonso and Hamilton in 2007, so for them I hope the Raikkonen/Hamilton (two world champions) combo does work. (Well, I don't, because I'm a Ferrari-fan, so I hope the Massa/Alonso combo workes out better than McLaren's one :D). And it also raises the following, less important, questions: Where's Heidfeld going to race, where's Kovalainen going to race, and what about Kazuki Nakajima?
There are also rumours about Toyota leaving F1 after this season, so -If Toyota leaves- that would make a total of 5 drivers looking for a seat (Heidfeld, Nakajima, Kovalainen, Glock and Trulli).