I’ve never been able to get Delicious to work with Twitterrific much to my consternation. Google searches proved fruitless as all I’d find were reviews that mentioned that Twitterrific could send bookmarks to Delicious. For posterity, here’s the tweet that spells it out.

The default is “http://delicious.com/save?url=”, you can use whatever you want (“tasty:” will work, for example.)
11:04 AM May 8th from Twitterrific

UPDATE: File this under “overthinking the whole damned thing.” It’s ready for Delicious bookmarking right “out of the box.” Seeing the grayed out text in the Bookmark field I assumed that that was the kind of thing I ought to be typing and that it wasn’t ready until I had. Had I never looked at the Advanced Settings page I’d have had no problems with Delicious, but I also wouldn’t have been able to use Instapaper either and that would have been a real damned shame.