Using a Custom Domain Name with Tumblr
I’m in the process of moving this site over from a self-hosted WordPress instance at Dreamhost to a simple Tumblr blog. Tumblr makes it easy to post from anywhere. That’s most important to me in light of the dearth of content here.
The DNS situation has been challenging, to say the least. This Tumblr support page was helpful, particulary in helping choose between an A-record or CNAME.
Also, be prepared to wait:
How do I know if I set up my domain properly?
After re-configuring your domain, you must wait up to 72 hours for the changes to take effect. When you visit the subdomain or domain, you should see a Tumblr error page — this means that the domain is correctly pointing to Tumblr, but that your blog hasn’t been configured to use it yet.