As far as I've heard the biggest thing is it frees up memory on your Treo. This is what Palm says from their web site:
What is the Treo 650 Updater 1.04?
New with version 1.04:
- Optimized memory handling with non-volatile file system improvements
- Improved user interface for multiple call scenarios
Also included from the previous version 1.03 (released July 2005):
- Latest version of Wireless Sync email application software
- Internal settings modification to allow correct access to Verizon Wireless roaming partner networks
I updated my Treo about 5 days ago and so far I'm not having any issues like others have.