Here's some information on Blackberry Simulation.
Part of what he is describing is application development but he is also doing web work as well. You will need to download an actual simulator and run it on your own computer.
From a practical standpoint, if you have a well-designed desktop site, your foray into building a good mobile web site should be a fairly straightforward process. If however, your desktop site favors table-based layouts, bloated HTML code, frames, and image based navigation, then I'm afraid your first mobile web site will be a whole new experience...
A good resource to convert your current site into a mobile device friendly format is Skweezer.com. Simply enter your website's URL, and it will display your website in a mobile-friendly version by removing large images, CSS styles, backgrounds and other page elements that may not display well on mobile devices. Use this as a template or starting point for your mobile-friendly CSS.