I just stumbled on BitNami, too, and tested it on my Mac with the built-in WordPress installer, and it works fine. I think it’s a good idea to use Bitnami, because you can install MySQL yourself under OS X and use it with the built-in Apache web server, but it’s a bit complex for a beginner. (You don’t need MySQL for plain html web sites, but you do need it for WordPress.)
So Bitnami makes it easy to do WordPress develop under OS X. Bitnami is called a “stack” as it contains everything that WordPress needs to run, like the MySQL that you would need to install yourself. But Bitnami also includes its own copy of Apache (the web server), PHP (the language) and other items in a self-contained package, so it doesn’t touch the OS X versions already on your iMac.
So get the BitNami – WordPress for OS X. It’s an almost 70 meg download, so if you’re not on cable or DSL, it will take a while to download. Or go somewhere that has fast access and download it on a flash drive.
Once you install it, let Bitnami start itself from the last installer dialog box, and when it’s running, bookmark the opening Bitnami page in Safari so it’s easy to get back to your new WordPress site. The newest version of Bitnami – as of 1/15/11 – also contains a control panel in the folder to restart the MySQL and Apache servers when needed after a reboot of your Mac.
When you want to move your site to a live server, you will need to move both the theme and images and also export database. Your theme will be in Applications/Wordpress-x.x.x/htdocs/wp-content/themes/. And see Moving WordPress « WordPress Codex.