There are a lot of questions regarding workflow capabilities in Office System 2007. We recently published a whitepaper that articulates what you get with WSS vs. MOSS vs. WF. Check it out:

Understanding Workflow in Windows SharePoint Services and the 2007 Microsoft Office System. (5Mb Word Format)

 

If you haven't done so, you should also check out this post on the SharePoint team blog by Eilene Hao that walks through SharePoint workflow with pointers to the ECM workflow kit.
 
I'd also like to introduce Paul Andrew. Paul, working with people across Office, drove this whitepaper. Paul's originally from NZ and the person to know if you're interested in Microsoft workflow technology. Paul recently delivered some great sessions at TechEd NZ and Australia... I had to stand in line just to talk to Paul... we hung out in Sydney on Thursday evening (24th August) w/ Fitz, Angus, Chris Johnson and Andrew Lowson to watch the Australian premier of "Snakes on a Plane". Fitz was very keen... so we all decided to join the fun. Paul and I didn't especially enjoy the movie... but it was definitely "different".