Search Engines And Password Protected Pages

I have a client who has repeatedly asked about getting a site-wide search engine for their website. There's arguments for and against, but I'm more worried about the practical aspects of the request.

The site is largely composed of password-protected pages. Member data is stored in an Access database. The pages are all secured by a basic include file that checks against session variables to see if the user is logged in. If "yes," they can see the page. If "no," they are redirected to a login page. Standard stuff.

One of my biggest arguments against the search engines is that since they can't get into the protected pages to index them, those pages will not be included in search results. Since the protected pages make up about 90% of the site, this pretty much renders the search engine useless.

Have any of you encountered a similar situation? Any thoughts on a way for the search engine to index those protected pages?

As far as the search engine goes, I was planning on cheating and using a form that sends requests to a simple google site search (it works surprisingly well on the non-password protected pages), although I'm open to other suggestions if you have any.