I fought with this for a few hours tonight so thought I would post it here for the hive mind.
I (perhaps foolishly) signed up for Google Apps and registered a domain name in the process. Google registers the domain with GoDaddy.com and properly sets up all the DNS records appropriately (I was happy to avoid that part). Registering for Google Apps standard is free, registering for the domain is $10/year.
For the moment, I really mostly wanted email services for this domain and don’t really care about the other Google apps services. I really wanted the domain itself to redirect to a blog hosted at WordPress (or Posterous … whatever). Those hosted sites both support buying a domain name and redirecting the name server records, but that’s not what I wanted.
Google apps / GoDaddy support adding a CNAME DNS record to point a subdomain like foo.bar.com to another site but the blog services don’t support a way to redirect the domain A record to their IP address, or at least this is non-obvious.
I eventually realized that GoDaddy support domain forwarding. If you enable domain forwarding, then GoDaddy will redirect via a 301 or 302 (permanent or temporary) your domain (http://foo.com) to your blog site (http://foo.wordpress.com). You don’t need to buy the domain service from your blog site for this.
You can also tell GoDaddy to enable domain forwarding with masking which will still show your main domain in the URL address bar. It looks like what that is doing is actually pointing some of my A records to the original site and some to the blog site.
In both the redirect and masking cases, all of the standard Google apps continue to work via the google urls like http://mail.google.com/a/foo.com, etc. However, I don’t think you’d be able to use the Google Apps web addressing to surface custom Google Sites urls. For me, that’s not a big deal. I gather there may be some SEO considerations if the identical content shows up on both sites. Not sure whether that’s real or not. I’m pretty sure I don’t care at the moment.