Best(cost and scalability) hosting solution for a web application

We're preparing a web application that realistically has the potential to quickly escalate to a large number of users. On the other hand, we would not be the first to be wrong in that assumption :p

We are doing this "pro-bono", so we would prefer to cut on initial costs - later maybe advertising can cover hosting - but at the same time we don't won't to compromise too much our ability to deal with the hoped/expected fast growth of our user community and therefore traffic.

The options we see are:

a) to start with in-house hosting and if we start getting a lot of traffic, then migrate to external hosting as quick as possible. However, I wonder how fast and easy such migration would be, since we wouldn't want to have the site down for 2 months while we migrate to a separate infrastructure.

b) immediately invest on external hosting, which would provide us the scalability, but require a significant investment from the start...

c) ?

I'm sure we're not the first facing this dilemma. Any suggestions? For those suggesting b), any recommendation on providers?

Thanks.

PS: The application will be GWT+Google Maps+Tomcat+MySQL.