Cloud Hosting Options

I'm currently working with a client who wants to launch a high
traffic, PPC/ad-supported site. The wisdom of this withstanding (I
mean, c'mon, dude pays his bills!), can anyone suggest cloud/platform
based rails hosting options that I can evaluate for this situation?
I'm aware if both Engine Yard and Heroku, and I strongly prefer
something more "paas" based than a simple virtualized setup, due to
the fact that patches and administration are handled for you
basically.

While I have every confidence that both Heroku and Engine Yard are
technically sound and financially solid (enough at least), are there
any other serious players in the Ruby PaaS hosting market space that I
can evaluate?

Please don't interpret this as a "which is better" discussion - I'm
only interested in finding the names of other companies whose services
I can evaluate for the project at hand. I'm more interested in the
right fit for the platform and its long term goals than what's
currently "in Vogue" that all the cool kids are using.

Thank you for any information you can provide!