I have had a dotCloud account for a while, but so far only to experiment with. dotCloud has announced new pricing models that look very developer friendly with free lower performance sandbox support and "live" support for production. They may have hit the sweet spot for supporting free development while nudging developers to not deploy small demo apps using their free sandbox model. I would love to see Heroku's and dotCloud's customer stats on free versus paid hosted web apps.
The basic idea is that as you add services you may by the amount of memory that those services use. I like their handling of horizontal and vertical scaling. I have not tried it yet, but from the documentation it looks like when you horizontally scale a persistent service like PostgreSQL, MongoDB, MySQL, etc. they automatically set up master/slave, replica sets, etc. as appropriate. You can also vertically scale any service by adding memory.
Another interesting PaaS that I have experimented with is Cloud Foundry that is, like Redhat's OpenShift, an open source stack that you can also install on your company's private servers.
Managing servers is a form of technical debt. Using PaaS costs a lot more for raw resources but "set and forget" web apps deployed to PaaS can end up being a lot cheaper to deploy over long term.