Hello.
We are looking for a new provider to host our Rails site -- we have outgrown our current provider (for the past 4 years) since our business is beginning to take off in a big way this year.
Which provider (e.g. Rackspace, Engine Yard, Heroku, Serverbeach, etc.) would you recommend to run a highly scalable site?
FYI, we currently have over 150,000 users that will likely grow to a million in the next year.
Thanks in advance.
There is a little company called Webappcabaret.com that specializes in PAAS. You may give them a try. They use the same cloud infrastructure as the others you mentioned, but I think their PAAS platform is far superior.
Ben Knight wrote in post #1016745:
Hi Ben, in response to your query you may want to check out BitPusher http://www.bitpusher.com/. I work for this company.
We do full-service managed hosting (infrastructure and operations) for Ruby-on-Rails and other web application stacks i.e. LAMP. Many of our clients have hit the "we need to scale but don't really know how" point and we help them through it.
In any case, good luck to you.
Hello Ben. Congratulations on your site's success!
I'd be happy to answer any questions you may have about my company, Engine Yard.
You'll not be surprised that given my bias, I think we're the ideal
solution for you. 
Seriously though, we've always focused on taking the pain out of running your Rails site for you, so you can focus on running your business!
-- Tom Mornini -- Founder & CTO -- Engine Yard, Inc.