If the blog has a BlogSpot URL of "myblog.blogspot.com", and it's published to "mydomain.com/blog", there's a "301 Redirect" for "myblog.blogspot.com", to "mydomain.com/blog". If you move to publishing to "blog.mydomain.com", "myblog.blogspot.com" will be likewise redirected to "blog.mydomain.com".
From the quick look there, any move from FTP to custom domain publishing should be transparent. And it is, if only the root of the blog / domain is considered.
If you use Permalinks, each post has its own URL, too. Now, we have complications. This post, published to an external server by FTP, might have a URL of "nitecruzr.net/blogging/2009/01/moving-from-ftp-publishing.html". If you're giving out that URL, you'll want that URL to refer to a similar URL when the blog is published to the Google server, as "blogging.nitecruzr.net/2009/01/moving-from-ftp-publishing.html". You're going to have to set this up on the remote server yourself though, as it's not a Blogger provided setting.
- If the blog is currently published as "mydomain.com/subfolder", so a post written this month is published as "mydomain.com/subfolder/2009/01/my-post.html", and you republish it under a custom domain as "subfolder.mydomain.com/2009/01/my-post.html" etc, and
- All posts are currently published as HTML, not PHP, on the remote server, AND
- You have DNS control of the server.
Then you MAY be able to use a "301 Redirect" of "mydomain.com/subfolder/" to "subfolder.mydomain.com". If you understand how to setup a "301 Moved Permanently".