With web startups it is probably more common that you will have a stack of 3rd party services you hook into of which may work in a way that someone else couldn't just plug their own credentials in and have it run.
Certainly not 'and have it just work', but I'm really skeptical that many web apps have such complex api dependencies that you couldn't just fudge a new one in.
Even app-engine apps have been successfully run using an isolated stack.
...and certainly most 3rd party APIs really wouldn't care; just another end user. Nothing special.
