Its safe to say that the general theme of Creative Suite 3 is "integration" and what a fantastic job they've done with it. In a way I see the March 27th announcement as the actual closure of the acquisition. Adobe and Macromedia tools now place nice together.
Thought I'd run through the product lineup and see what features make it worth the upgrade for me.
Photoshop CS3 (Extended) Although primarily aimed at specific industries, I would go for Photoshop Extended for the video, 3D and advanced imaging support.
Dreamweaver CS3 AJAX support, better CSS management, improved browser compatibility check
Flash CS3 Photoshop and Illustrator import, copy motion, component skinning, AS3 debugger
Fireworks CS3 Prototyping designs with web controls, export to MXML
Illustrator CS3 Live color, points and path enhancements
Contribute CS3 Blog editing, Firefox plug in, FLV support
Acrobat 8 Professional PDF forms (results emailed back to you), PDF packages
These are obviously just the Web Premium CS3 products, from what I've seen the video editing products will also be well worth purchasing an upgrade for but am yet to look into those in some more depth.
If I was to pick one product that has the most compelling case to warrant an upgrade, I'd say Photoshop Extended but in truth all products have had such significant improvements in terms of integration that you'd be a fool not to go with the entire suite.