ISS also provides a managed service for this both directly as well as through resellers. I would guess they have broad geographical coverage and high quality of service though we do not use them. It sounds like your client has a healthy budget and may be interested in top-tier service.

http://www.iss.net/products_services..._intrusion.php

I would be interested to hear the results of your search and what interesting lessons came to light about this area as I am going to be looking into the same issue soon.

Good Luck!