Webmasters get anxious when they move a website for fear they'll lose visibility in search engines. I'm glad to say that in my move the three major search engines all did a great job following my blog to the new URL.
The key thing that made this move work was getting the 301 redirects right. When you move a web page the old URL should still work, but instead of serving content it should return a 301 Moved Permanently pointing at the new URL. I did this with Apache mod_rewrite, specifying the flag [R=301]. It took a bit of fiddling to redirect both hostname and path in a single response.
With the redirect in place everyone quickly started indexing the new URL. The first search engine to find the new host was Yahoo, which had 47 pages for my new hostname in less than 15 hours. Google had the new site in about 24 hours. MSN got it in about 48 hours. Now, 2 weeks later, all three engines have a fairly good collection of my pages at the new URL. Google and MSN now favour the new hostname but Yahoo still has the majority of pages from the old name.
I'm also curious about when the search engines start forgetting the old host. Google decided my canonical URL was the new host in about 36 hours. MSN switched in about 3 days, although it took awhile for the new URL to float to the top of the search results. Yahoo took about 12 days to switch. Of course with my redirect in place, people find my blog either way.
So what's the bottom line? 301 redirects worked for me and all the major search engines did a good job of following mine. Search engines are complicated; it's possible something I haven't thought to check didn't work. But folks can find my blog at the new URL, so I'm happy. And it's pretty amazing how fast this update happens. My blog is only one of billions of web pages out there; search engines are able to track changes like mine across the entire web in just a few days.
PS: while I work for Google, these notes are something I did on my own time out of curiousity and do not reflect the work or views of my employer. And no, I don't get special treatment by Googlebot. I wish!