As a multiple WordPress administrator (I control 10+ WordPress sites), I use the WP contact form plugin written by Ryan Duff a lot. Most of the times these contact forms are used as some form of conversion on these sites. This means you want to know as much as possible about the person sending you the message.
The two most important things for me are the page from which the person came to the contact page (this is the page that urged him to send you a message after all), and what he came from in the beginning, be it a search engine or another site. When it is a search engine he came from, you’d want to filter out the keywords so you don’t have to start deciphering the query string.
So I’ve adapted Ryan Duff’s contact form to do just that, and I proudly present: the enhanced WordPress contact form.
This sort of thing is very useful towards your client too, as it raises the awareness of which keywords people use to find them.