Microsoft provides a white paper showing some differences between Windows XP with Service Pack 3 and Windows Vista with Service Pack 1:

Brief Description
This white paper compares the following features and capabilities of Windows XP with SP3 and Windows Vista with SP1: security, management, deployment, mobility, and productivity.

Overview
This white paper helps customers compare Windows Vista advancements with Microsoft Windows XP Professional. Using this comparison, customers can adjust their expectations for the security, management, deployment, mobility, and productivity of either operating system. For each feature or capability, each section compares key Windows Vista advancements against Windows XP.

Versions of the document are available in XPS and PDF formats.

I haven't looked at this yet, but I'm nervous it's going to duplicate some of my findings from the upcoming "XP Good Enough" part 2 article....