i'm looking for a web based tracking system for tracking my company's
engineers. The system would have a client interface, a customer
interface and an engineer interface via a PDA. Ideally the system
would be charged using an ASP model, e.g. 100 bucks per engineer per
month.

You might like to have a look at OpenAir.

From the blurb on the web site: http://www.openair.com/home/

OpenAirŪ Professional Services Automation (PSA) software helps
organizations dramatically increase profits by better managing
critical business functions and identifying, measuring, and improving
upon the key metrics that drive profitability. It provides sales force
automation, project estimating, resource planning, project management,
project accounting, knowledge management, time and expense tracking,
purchase orders, client billing and business intelligence in a single,
integrated, Web-based system.

I have no affiliation with the company.

Cheers,
tcm-ga

You must be find your need at www.isputil.com.

in the product list look for ISPUtil application.

good luck.