If you do, or plan on doing a lot more mobile integration in the future, then getting both Dreamweaver CS6 and Fireworks CS6 (Fireworks CS6 New Features Overview from Graphics Software @ About.com) is a good idea. The mobile features are much easier to use and there is much tighter integration with PhoneGap.
The user interface hasn’t changed much in Dreamweaver CS6 and that includes updates for things like CSS3. While CSS3 is supported, and you can use browser prefixes, the CSS panel hasn’t changed much even with the new CSS transitions panel. It is helpful to have the CSS3 properties included in the drop-down menus, but they aren’t in the panels yet, probably because they haven’t been standardized.
If you’re upgrading from CS3 or earlier, then you will love how easy it is to work with the sites now. This was added in CS5, and it is a very useful tool. You don’t have to have all the information (like FTP accounts and URLs) to start building a site. And you can include multiple servers for doing testing and integration. Also the templates that were added in CS4 are very helpful, and they were all improved in CS5 to add even more comments to make them easier to figure out.
If you are a web developer, focused on the code, there wasn’t a lot that changed for you. Most of the new features surrounded grid layouts and doing more in design view.