Does anyone use or know of a web filtering software for Treo600/650s?

Background - We developed a PHP/MySQL solution for our construction company which can be accessed by a Treo 600/650.
Good News - We are saving big bucks on labor and material now.
Bad News - Constructions crews can be easily attracted to the bzillions of adult websites or whatever.

I am looking for a web filter which will help me lock them in to our company website or our suppliers' web sites.