I ran across some ideas for using social software in libraries on the BIGWIG Social Software Showcase, an online unconference planned around the time of ALA.
I thought the description of "Library Thing for Libraries" was especially interesting.
For example, examine this catalog record for Bridget Jone's Diary in the Danbury Library catalog. "Other Editions and Translations" are added to the catalog record (you can see a sound recording of the book & a version listed in Spanish). If you scroll down the record, you will see suggestions of "Similar Titles" and "Tags." This data is pulled from Library Thing. This might be more relevant for public libraries, but could become useful for academic libraries as well...
I think that WIT is planning to give patrons the opportunity to add reviews of books to our catalog. I look forward to seeing that feature of the catalog in action.
Could our catalog provide spell check features, such as "Did you mean x?"
Could information from the Internet Movie Database (IMDB) or a link to IMDB records be added to catalog records for movies?
I know that some of these features are not technically possible yet for OPACs, but what do you dream that our OPAC could do?