www.secureworks.com
(disclaimer:I work for Dell) Dell acquired these guys over a year ago and seem to be rated highly in the Managed Security industry. In addition to managed security they also do consulting and testing. They are a separate division then I work in so I don't know a whole lot about them, but I thought I would throw them out there. A lot of the financial clients I talk to have them for managed security and rave about them. I also don't know how much they cost for just assessments/testing. As stated before they are in a separate division from me so I wouldn't be getting compensation on any of this, I just know they are very highly thought of in the industry.