One of the big complaints about google's App Engine was that they were trying to induce some sort of vendor lock-in by requiring the use of their python libs. After having stumbled through the getting started guide, I don't see what the foundation for that complaint is.
The way this works is that python has something called the WSCGI (google around for the spec for more info). This basically allows you to route web server requests to different portions of your python code. It's kind of like having "dispatch.cgi" embedded into Apache for a Rails application. From a programming standpoint, you simply route URls to functions, and since WSCGI is a python community standard it's the same thing that stuff like Django uses. Other than support for BigTable (which I suspect can be ORM'd out of the way) and gmail account authorization (which you can sidestep if you want to do something like run your own LDAP service on a remote server), there's not a whole lot in there that's trying to pin you down to google.
The only thing I see that's going to trip up the rails and php folks is the lack of anything similar to WSCGI in those communities.