looking for some real world performance numbers

I'm working through the architecture design for a new product.  We
have a small group working on this.  It's a web app that will be using
ruby on rails.  The challenge I'm running into is that the latest
conventional wisdom seems to be that since obviously databases don't
scale on the web, you should just not use them at all.  I have a group
of otherwise very bright people trying to convince me that a rdbms is
not a good place to store relational data because eventually it won't
scale.  And of course we don't even have version 1 of our product out
of the door.  I'll admit we do have a very good chance of actually
getting tons of traffic, but my position is to use a rdbms for
relational data, and then if and when it won't scale any more, deal
with it then.

So what would really help me is some real world numbers on how
postgresql is doing in the wild under pressure.  If anyone cares to
throw some out I would really appreciate it.

Chris