For quite awhile now, del.icio.us has offered JSON support, but not a proper JSONP implementation. I even wrote my own proxying script to fix this oversight. It seems that that will no longer be necessary. If you pass callback=CALLBACK to a del.icio.us JSON feed it will now output proper JSONP that does not set the Delicious.posts variable (easy to fix in your callback function) but calls CALLBACK with the JSON object as a parameter. Compare these:
http://del.icio.us/feeds/json/singpolyma.techblog/
http://del.icio.us/feeds/json/singpolyma.techblog/?callback=CALLBACK
There has, as yet, been no official announcement on the del.icio.us blog, so this feature may yet be pulled like some others have, but I hope they will not. Hopefully I will soon be modifying my version of FreshTags to make good use of this native JSONP support.