What kind of integration are you interested in? You might look at the biblio module, whose author has talked about some related issues (like porting the Zotero citation formatting code to PHP).
I'm interested in exposing the metadata for the web versions of the articles in the magazine I co-edit, www.peaceworkmagazine.org, to users of zotero. The site is powered by Drupal (currently version 5, though we hope to do a re-theme (visual redesign) and port to 6 this year).
I think thatit will include RDFa (which Zotero should both produce and consume).