The answer appears to be no, which surprised me. The authors, from Southern Illinois University, do caution that it's a pretty small sample and not to draw many conclusions, but the evidence of their transcript logs seems to indicate that providing a co-browsing option along with chat reference doesn't really add much instruction to the transaction, compared to a similar number of sessions where co-browse wasn't an option. In exploring options for providing IM reference here at the U of C (finally!) I happened to be chatting earlier today with a librarian at an institution that offers both IM and commercial chat with co-browse, and was told that IM pretty much gets the job done and that they rarely feel the need to co-browse, so there's some anecdotal evidence to go along with the article.
Title: Instruction via chat reference: does co-browse help?
Author(s): Stephanie J. Graves, Christina M. Desai
Journal: Reference Services Review
Year: 2006 Volume: 34 Issue: 3 Page: 340 - 357
Technorati Tags: IM, chat, reference, co-browsing