Google just introduced version 4 of Google Toolbar for Internet Explorer including support for centralized bookmark tagging and notation from multiple computers. The new feature allows any toolbar user with a Google account to store bookmarks within their Google search history for synchronization and editing. Google bookmarks can also be added and edited via a web interface.
You can add a new Google bookmark anywhere on the web when logged in by passing Google a few URL-encoded values.
http://www.google.com/bookmarks/mark
- title
- the title of the bookmark
- bkmk
- the bookmark URL
- labels
- comma separated labels or tags
- annotation
- A note or description about the bookmark
- s
- 22 character user token
To edit a bookmark just resubmit the original URL with the new values. You should receive a response including a 
star graphic if you are successful.
It should be possible for third-party applications to use Google’s bookmark storage system once you obtain the appropriate token.