I'm surprised MS jumped so early, I'm guessign their hands were tied, either someone else sniffing around, or a need to push for more attractive Windows phones. I always said they would wait for it to be worth nothing.
Nokia peeked with the 6310i. Everything that followed was half baked. Symbian was a crock, and they knew it, hence spinning it off into an open source setup, which just created more problems. Until the Lumina series, Nokia never managed to produce a useful smartphone. It was a market they felt they didn't need to be in, until they realised, way too late, they did. Then they rushed, and made a pigs ear of it, producing a raft of unfinished junk.
They were never going to be competitive in the Andriod space, so were forced down the Windows Phone route, they were out of all other options. Unfortunately, Window's (desktop) poor reputation, mostly undeserved to be honest, hurt the Windows phone market. Shame, as MS did make the first usable Smartphones (Nokia Communicator wasn't really a smartphone, just a PIM with a built in mobile), and opened up the market, which was better explioted by Apple and Google.
It would appear that Nokia derailed due to lack of control over all the various projects that its engineering teams were working on, and with no cooperation between those teams. At one point, they were maintaining the mess that was SYmbian codebase, and 2 Linux based OSes, and the handset engineers were not really able to liase with the software teams, so everything was delivered late and unfinished.
It was inevitable, but I'm still surprised MS didn't wait.
Without doubt, RIM will follow suit by this time next year.