Nokia never went with Android, because of a few reasons imho...
- Most importantly, Android (especially in the Froyo days when they chose WP) is a messy OS, even more fragmented than Symbian became. It is clunky, geeky, and has about as much simplicity to the average consumer, as the workings of an internal combustion engine. Nokia wanted a simple, closed, reliable, and standardised OS, anything less does not suit their customer-base.
- Microsoft will do just about anything they can to ensure that WP is a success, and like it or not, they are in the game for the long-haul, and will not only use their own expertise and resources to develop the best OS ever, but they will also invest freely in the platform. The future of Android is less certain, due to it being open-source, and the development to make it the best OS ever, will never be so much of a priority.
- Nokia maybe had the forsight/an idea, to consider that Google were circling Motorola, or at least aiming for some takeover of a hardware vendor in the future. The takeover of Motorola by Google, has imho, ended any chance completely, that Nokia may be interested in utilising the OS someday. Nokia know/knew Microsoft are a software company, and have little intention/interest in making hardware (Xbox aside). They always use 3rd party companies to build their phones, and simply create the core-software (WinMo devices, KIN, WP7 upon release etc).
- It's likely that Nokia got a better/preferential deal from Microsoft, maybe with cash for marketing, preferential treatment, lower license costs etc... Android and Google would likely never offer such perks.
- Also, for those who consider Android to be some glorified cash-cow or something - take notice of how successful it has been for Sony/Sony Ericsson.