Just ran into an issue where the built in Media Uploader is not appending the base URL of my blog to the URLs of files I upload with it.
When I upload an image, the File URL appears as...
http:///wp-content/uploads/...
Since there are three slashes at the start, i can only assume that there is something killing the site url from being written. I did move this site from another server (the site is not live yet, and is still in a testing/development phase).
update: upon further inspection, the GUID of the uploaded images DOES show the full URL, rather than http:///, it shows http://mydomain.com/ at the beginning of the URL.
I am thinking now that it might be an issue with a plugin. Ill post another update once i find which one it is. Weird that it is blocking it from within the preview section of the Media Uploader after the image is uploaded too.