1. Choose Android and be delegated to one of the hardware supplier amongst HTC, Samsung etc (not necessary the preferred one).
2. Stick with Symbian, Meego and be slower and further down road (Users don't care about the OS. Just whether it works. I would argue that developing OS internally is just playing catching up with minimal hope of overtaking the others significantly)
3. Try Microsoft and hopes the strengths compliment one another (not-too-bad windows phone 7).
I don't envy Nokia. It's a tough choice. Effectively, they are trying to move into a field just disrupted by Apple and competing on a sustaining basis. Doesn't bode well but what other alternatives do they have?