I have a domain controlled with google-apps. Now I want blog.mydomain.com to be a wordpress blog. How do I do that?
I think you have 2 options both of which require you to use Google-apps support for mapping your domain to a google appengine application.
You can use Google Appengine to run Wordpress (it's a bit of an advanced technique) with your Google-apps and assign blog.mydomain.com to your google appengine application.
This blog describes the modifications you need to make to wordpress to run on Google's Datastore and to run PHP through the Java runtime:
You can see it running as Java Google Appengine application:
Another alternative might be to use Google Appengine as a reverse proxy, mirroring your wordpress hosted site (Google appengine would allow you to handle massive surges in traffic and comes with huge free quotas).
I have my mail, calendar etc in Google Apps, but I maintain the DNS from outside this (I didn't know you could do it any other way).
So I have: