I just finished helping someone with an issue where they were trying to map blog.companyname.com to show their blog at companyname.textpattern.com. This is very similar to what I did with custom domain mapping with WordPress.com and GoDaddy, except its with TextPattern (not WP) and was a subdomain not a domain itself.
Here was the process that worked for us in this case:
0) Go through the wizard for “Domain Mapping” in TextPattern. Control Panel > Site Access > Domain Mapping. If doing a subdomain from your site to your TextPattern site, than put your entire blog.yoursite.com as the “domain” to map to yourblog.textpattern.com.
1) Delete anything you have in GoDaddy that relates to domain masking, subdomain masking, etc. Step 2 will take care of all of that. Make sure there is nothing else that will interfere. It could also be called “domain forwarding” or something. Edit: It is called Domain Forwarding
2) Delete any A records for “blog”. Do this even if you already called GoDaddy or TextPattern support and they told you to do this. They are wrong. This is needed in GoDaddy for domain Forwarding but you don’t want to do that. For mapping a CNAME subdomain you need to delete the masking (step 1) otherwise the forwarding takes precedence and your CNAME will never kick in.
3) Create a new CNAME entry. It has two parts. Where is says “host” put “blog”. No http. No www. Just “blog”. In the second part where it says “points to” put companyname.typepad.com. Again, no http. No www.
4) It may take up to 24 hours to kick in, but it usually does with 3-4 hours. Should be working by tomorrow morning. You may need to reboot your PC though for it to kick in as sometimes browsers cache the old method even if you try to dump your cache.
In this case, it was actually up and running within about 30 minutes. Good stuff. Many thanks to this article which was hard to find on the TextPattern site, but was helpful.
Also, if you do this and do the wizard part in Step 0, put “blog.companyname.com” as the “domain” early on in the wizard. Otherwise it will try to map your root domain and that won’t work if you are trying to map blog.companyname.com to your TextPattern blog.
Lastly, don’t let anyone at GoDaddy or TextPattern tell you that you can’t have it mapped the way you want it. You can. Its just not something that they are really familiar with. You can have your TextPattern blog (or your WP blog too I believe – not sure about the subdomain though on WP but 99% sure) “live” at blog.yourwebsite.com and keep that URL such that no one ever sees the something.textpattern.com version. Better for SEO. Better for Branding. Better.