Well, you’re pretty much on your own for that :)
I think that Delicious Library should definitely include this export option; it is a great application to manage your books, CDs and DVDs (and even games!). On the other side, I use a lot LaTeX for my Master degree’s papers, and being able to export my own library to a BibTeX file is something that really helps.
In the meantime, I created a Ruby script that reads your Delicious Library XML file, and outputs a list of BibTeX entries. Once exported, you can use BibDesk or TeXShop to open and edit the file, and use it for your papers and reports (as I do).
Here’s a sample BibTeX file with some classics in my own Library.
Hope that you find it useful! Feel free to use it and, as usual, I’m not responsible of what might go wrong with it!