Update: Well, that didn't go so well after all. The feeds don't seem to follow the migration, so if you move a blog the old feeds don't seem to work. I'll see if I can get some Google tech support help, but in the past they've been hard to work with.
Update 6/20/07: I was too impatient. A post to Google Groups (I didn't try Google tech support) received a very kind response from "wasted":
I don't use bloglines myself, but Blogger will use a 301 redirect on the blogspot address to go to the new custom domain, for everything including feeds. So any feedreader should follow the redirect...And so it does. It just takes a bit longer to update than usual, and it follows the common bloglines behavior of showing an unread count of "10". Bloglines updates the URL as well. I did two roundtrips on my test blog and it works. I've switched back to the old urls (hey, I was justifiably nervous) but I'll switch them one at a time over the next few days. It's noteworthy that bloglines managed two roundtrips, a "301 redirect" is "permanent". (see also)