I know what it's like when a new feature arrives: you want to try it out right away. Before you run out and register a domain for use with your WordPress.com blog, there are some key things to know about domains and what our system does. I'll try not to get too technical about it.
Many of your questions on this topic will be answered in FAQ pages and I may update this posting as well. Ask all you like, but please do not offer commentary or answers on this topic. I will be hovering over the Delete button on this one! :-) Also, I may delete a question once I answer it in an update. Thanks in advance for asking!
So, here are the basics. Every WordPress.com blog starts off as a subdomain of wordpress.com, such as andy.wordpress.com. That subdomain will always be attached to that blog. If you decide to get a new domain for your blog, such as andyskelton.com, your old URLs will simply redirect to the equivalent URLs on the new domain.
The same redirection will apply to your feeds but you might want to tell people to update their subscriptions. Most feed readers will happily follow the directions to the new URL but some might not.
You will be able to use any domain that you already control; you don't have to register with us. Many people will be happier with other registrars because they offer fine DNS control. If you register a new domain with us, your domain will be usable for your blog only. You will be able to transfer your domain to another registrar, though not in the initial release. Also planned is the ability to register privately, but until that is released your info will be shown in
Domains registered elsewhere must have their DNS properly configured before they can be connected to a blog. Detailed instructions will be made available but we cannot support the various tools provided by registrars.
We do not have any plans to offer email hosting. We may offer MX editing so you can host your own email server elsewhere, but that will not be in the initial release.
Domains registered elsewhere can receive email if you host your own email and DNS and put your blog on a subdomain. Your blog domain will not be able to receive email--at least not in any configurations we will support.
You can have many domains assigned to one blog. Only one domain will actually host the blog. The others will redirect visitors to the primary domain. You can switch primary domains any time, but I recommend against frequent switching.
Now, about your Google Juice: it will probably be affected for a short time. I don't know how quickly a redirected website will gets all of its mojo back from the move, but any search engine worth using should have no trouble respecting the move.
Until the search engine indexes your new URLs, all of the old URLs in the search results will still work. Visitors to old URLs are redirected to the equivalent new URL.
Your old XMLRPC gateway should still work.
The initial release will not allow you to use a subdomain of a domain that is already connected to a wordpress.com blog. So if you use hill.com or jack.hill.com you cannot use jill.hill.com. In the near future it will be possible for the owner of the first hill.com blog to authorize other hill.com blogs.
Unused subdomains will be redirected by removing subdomains until a known domain is found. In keeping with the examples above, http://www.andyskelton.com and blog.andyskelton.com and a.b.c.d.andyskelton.com will all redirect to andyskelton.com. If I had used blog.andyskelton.com as my domain, http://www.blog.andyskelton.com would redirect but the behavior of http://www.andyskelton.com would be undefined.
If you read this whole thing and you see me in a bar, I'll buy you a beer! :-)