It looks like I'll need to provide an update to my original comparison of the Segoe UI typefaces. According to Long Zheng and David Warner, the version of Segoe UI coming in Windows 8 has seen some subtle improvements.
Here's a brief sample from I Started Something:
Looking at my original comparisons below using Segoe UI 5.01, you can clearly see how some of the traits of Segoe WP are making it into Segoe UI 5.16.
