A couple of comments...

I don't understand why everyone gets hung up on this. GAE provides everything you need to do your own authentication, so you have a choice. Either use their authentication system, to save time, and accept the closer ties with Google. Or write your own authentication and remain (more) independant.

I don't worry too much about Google's pricing - in fact I'm looking forward to the day when they start charging, because then the quotas should disappear. I think Google built GAE to get more of the internet on their own servers, so they can spider it easier, and reduce the risk of ISPs starting to charge them for delivering traffic. They will make some money off it too, but if they put prices up, they'll drive people back to Amazon or their own servers, so they can't get too unreasonable.

Lastly, one risk you haven't mentioned is service levels, particularly during this preview period - and we don't know how long the preview period will last. I have an application I'd love to migrate to GAE, because it would take the sysadmin worry away, but major banks use the app and I can't risk any outages. This will come with time - hopefully Google will give us an SLA, but we'll also get a pretty good idea after a few months of use.