Great question Chris!
I guess there are two things you can do:
1) Try and incorporate similar elements into your portfolio website.
For example, if one of your major accomplishments at work was to create a database-driven application using PHP, then perhaps you could recreate an example of this on your website - without any of the branding or sensitive information.
This will show off your capabilities to a potential employer, but it'll also highlight the kind of project you'd be capable of pulling off in a very practical way.
You won't need to rely on a database of sensitive information, and you won't have to spell out how the the app is used internally in your current company. A good recruiter should be able to see how your skills would be applicable to their model regardless.
2) Be honest and explain why your online portfolio doesn't include work samples.
Let's say you're afraid to post examples of your work online in case someone copies your client's work.
Simply throwing a line saying something like:
"Due to confidentiality and contractual obligations, my portfolio is only available for review in person."
will go someway to explaining why specific work samples are missing from your portfolio.This shows two important things: