My first impression is that it does provide a lot of content links on one screen, but it will be quite difficult for a user to remember which grid to click and navigate to that grid. That grid layout somehow reminds me the Million Dollar Homepage 
T-Mobile now provides Windows Mobile 6 update for Dash and MDA device. After updating you will need a little patience for the first boot up and wait for the “Cool Boot Config” screen for some final configuration. Other than that the firmware update itself is very smooth and takes only a few minutes.
Windows Mobile 6 Product Reference Guide [PDF, 11.5MB] is a good document for complete feature list. Besides the HTML email support and fixes of a few annoying bugs, here are a few differences I noticed immediately:
- Windows Live services (Live Mail and Live Messenger) are more integrated in. When you launched IE, the default page is a Live search page
- Live Search for Mobile application is pre-loaded
- Microsoft Office Mobile and Voice Command are included.
Some links about UMA:
WSJ.com: How Wi-Fi Can Extend T-Mobile’s Range
solsie.com: T-Mobile User will Have “Cell-to-WiFi service” (UMA) Soon
A podcast from Mobile Computing Authority: UMA from the source
Open Source Software for Mobile Phones