I am not sure if an app running on a different cloud platform like (Google App Engine / Azure / etc.,) can make use of Database.com and still provide decent response time to user requests.

This service will be helpful for pre-generating reports having queries which span across multiple tables (JOINS) and which can use aggregate queries (SUM/COUNT/MAX/MIN/etc.,.), both of which are currently not possible with Google App Engine's BigTable. But with Google already working on full-featured SQL database support (http://code.google.com/appengine/business/roadmap.html), I don't think I will use it for such an use-case too.