In the bad old days of computing you needed to put code into escrow if you lic'd it to insure that you had access to it in the event of a bankruptcy or the company just closing down, etc. With commercial products that also release to an open source project like Android there isn't as great a need, perhaps no need.
I think the bottom line here is that in the short, med and prob. long term it's not in Google's interests to play favorites and if they do Samsung and HTC (among others) have the resources to Fork Android continue on that path free from Google. They can do this at any time.. and I think that puts reasonable pressure on Google to behave
In terms of MSFT and Mango and future releases one has to not only consider the quality of the OS but also the cost of any licensing fees as well as it's popularity in the market place, it's performance, etc. Then even if MS doesn't buy Nokia there is certainly a tight relationship there and MSFT can certainly play favorites perhaps more easily than Google can.