This is the very anticipated Update 1 for Dynamics NAV 5.0 SP1. Not only does this update resolves various issues but also makes Dynamic NAV 5.0 SP1 client Windows 2008 compatible.
I highly encourage that you install this on any site which already has NAV 5.0 SP1 running. This update solves some critical stability issues with Dynamics NAV Application Server.
You can view the complete list of issues resolved on Partner Source knowledgeBase or Direct Link:
Microsoft Dynamics NAV Support Professional
EMEA Customer Support & Services - SMS&P
Microsoft UK Ltd