Google is personalizing the Google experience: personalized search, personalized home page, blogging, etc. How long before they jump into personalized bookmarking?
I think Google could become a formidable opponent to del.icio.us and other social bookmarking services (or more aptly, it could become a formidable assassin) by adding a tiny button or checkbox beside each of its search result items.
Using del.icio.us as an example, each of its bookmarks' four data fields could be filled automatically: 1) URL, 2) description, 3) extended (filled by the short snippet of text Google displays for each search result), and 4) tags (defaulting to the search terms you used to google your target).
I click the checkbox, Google ajaxes a database or sets a cookie. A thin, unobtrusive tag field pops below the search result with the default tag(s) present, allowing the user to add or modify these tags. As I type these changes, Google again does a bit of ajaxing and/or cookie-setting (a kind of reverse Google Suggest - updating a database rather than selecting from one.)
Whereas del.icio.us and company are slowly building enormous databases of links and tags, Google already has the mother database sitting on its servers. All they need to do is create a new table, containing a user_id field, a URI_id field, and a tag field (user johnsmith has bookmarked http://www.commonnames.org with tags "common names").
Of course, it may not be as simple as this, but it doesn't appear all that difficult.
Google, on your bookmarks, get tagged, GO!