[po]

A Couchapp has features that are trivial in couchdb (replication, map-reduce) but require more setup in traditional server side code. The target market will be applications that value these tradeoffs.

If you decide you need something else, I don't see the problem with having some parts of the application served by couchdb and some served by node. Put it behind varnish, cdn the static content, add a queueing system or whatever...

I think a couchapp is a decent way to prototype something out and if it goes viral you're going to be rewriting everything anyway no matter what system you use.