I was recently asked by one of our clients to evaluate how hard it would be to redesign their website to work on mobile browsers (phones, pda, blackberry's). One of the first things I thought about was being able to test what I would be working on. I found a few things to do the job. One is an online tool made by Opera, another is a free download from Winwap Technologies.
This particular site I'm working with is an older table-based design (not done by me) – and I'm thinking that switching it over to be CSS-based and then modifying the CSS for mobile users would do the trick.
Also – here's a good article on mobile web design from ALA.