We've been logging all keyword searches on our OPAC for nearly 3 years and now have details for over 3 million searches. Just in case the data is of any use to anyone, I've uploaded an aggregated XML version to our web server: http://library.hud.ac.uk/data/keyworddata/
As with the usage data, we're putting it out there with no strings attached by using an Open Data Commons Licence.
The XML file contains a list of about 8,500 keywords. For each keyword, there's a list of other terms that have been used with that keyword in multi-term searches. The readme file contains more information about the structure.