Update on 12/18/2010: I've updated the tool to handle the V2 Delicious API.
For a while now, Kushal* had been wanting a way to import his del.icio.us bookmarks into Google. There are some advantages to this, including the fact that your
tags labels show up in search results if they contained a bookmarked site and one-click Toolbar access. It was pointed out to me that the bookmarks site has a POST/XML-based bulk import interface that the Google Toolbar uses to import IE Favorites. I initially thought that I could use del.icio.us's JSON API to get the list of bookmarks, thus having a completely client-side solution that did not require the user to hand over his/her credentials. Unfortunately that turned out to be limited to 100 posts. Hack-ish workarounds came to mind, like getting the list of tags first and then getting the bookmarks for a particular tag (still wouldn't work if there were more than 100 bookmarks for a single tag). I abandoned that approach, but for the sake of posterity, that version is archived here.
Since del.icio.us also has a REST API that allows full access to data, my next thought was to use that. pydelicious is a pretty simple to use Python library for accessing the API, and it was easy to create a script that used it to fetch the list of bookmarks for a user. Since I already had the code from my previous attempt that took the JSON output and uploaded it, I merely made the Python code output similar JSON and used the same upload method.
This approach seems to work pretty well, but it has the caveat that the user has to enter their username and password into a third-party site. I've provided the source code so that the paranoid can run their own copy. While the Google bookmarks site doesn't have an API per se, it does export its data as RSS feeds thus this does not have to be a one-way transfer; an enterprising hacker could write an importer in the other direction.
* His newly-released luxur(n)y site is interesting if you're in the market for a NYC luxury rental.