Move photos from Flickr to Picasa Web, Photobucket, etc. with Migratr
Other than sounding a bit like an angry person looking for their cheese grater, and moving images around the Web, that's about it. It preserves meta data, descriptions and tags -- and even album organization. What I can't tell you is whether it works equally well across all of the photo sharing services, but there's a support forum if you need some help getting things working. For me, it was simply a matter of authenticating with Flickr and Picasa... and that was that. Easy.
You can import/export from/to: 23HQ, Flickr, Gallery2, Picasa Web, Phanfare, Photobucket, SmugMug, Zenfolio and Zoomr. Are there any other large services on the Web? Expect to see support for them soon, if so!
Looking at the files it creates, it simply downloads every image and stores all of the relevant details in an XML file -- so you don't need to worry about being 'locked down' to some weird storage format.
Migratr is exactly what you need to escape one photo dump and join another -- download it. You may not need it right now, but one day you might!