Blaine recently posted that there is a CAB to Prism Comparison Available on Codeplex. This links to Composite Application Guidance for CAB Developers, a download from the CompositeWPF community site.
While this document does not help you decide IF[*] you should switch from CAB/SCSF to CompositeWPF, it does help with HOW, the differences between the two coding models, where the WorkItem went, how to emulate the behavior of WorkItems, and a few other interesting things. If you are a CAB developer considering the transition to WPF, you should definitely read this.
[*] Deciding which to use should be fairly simple: If you already know CAB/SCSF and you need to work in Windows Forms, stick with CAB/SCSF. If you need to host some WPF widgets in a Windows Forms environment, use CAB/SCSF. If you need a full WPF stack, use the Composite WPF guidance. If you are not constrained by the technology choice, things get tricky and the decision should factor in a whole bunch of factors including (but not limited to) which framework you are more comfortable in, which your team is more comfortable with, and where you see future investments in technology going. For each team/group/organization/company this is a different decision with different factors to consider.