[Update: Migratr downloads all your files to the desktop, with your metadata in an XML file, so it's a great way to backup your content if you're feeling a bit nervous about the sustainability of the online services you use. If it's saved your bacon, consider making a donation.]
This is just a quick post to recommend a nice piece of software: "Migratr is a desktop application which moves photos between popular photo sharing services. Migratr will also migrate your metadata, including the titles, tags, descriptions and album organization."
I was using it to migrate stuff from random Flickr accounts people had created at work in bursts of enthusiasm to our main Museum of London Flickr account, but it also works for 23HQ, Picasa, SmugMug and several other photo sites.
The only hassles were that it concatenated the tags (e.g. "Museum of London" became "museumoflondon") and didn't get the set descriptions, but overall it's a nifty utility – and it's free (though you can make a donation). [Update: Alex, the developer, has pointed out that the API sends the tags space delimited, so his app can't tell the different.]
And as the developer says, the availability of free libraries (and the joys of APIs) cut down development time and made the whole thing much more possible. He quotes Newton's, "If I have seen further it is by standing on the shoulders of giants" and I think that's beautifully apt.