Here is interesting information that the Exchange Team has produced:

Last week we released Exchange Server 2010 Service Pack 1. It has received some great feedback and reviews from customers, experts, analysts, and the Exchange community.

The starting point for SP1 setup/upgrade should be the What's New in SP1, SP1 Release Notes, and Prerequisites docs. As with any new release, there are some frequently asked deployment questions, and known issues, or issues reported by some customers. You may not face these in your environment, but we're posting these here along with some workarounds so you're aware of them as you test and deploy SP1.

1. Upgrade Order
2. SP1 Prerequisites
3. Prerequisite download pages linked from SP1 Setup are unavailable
4. Missing Exchange Management Shell Shortcut
5. Upgrading Edge Transport on Forefront Threat Management Gateway (TMG) and Forefront Protection for Exchange 2010
6. Static Address Book Service Port Configuration Changes
7. iPhone, OWA Premium and POP3 & IMAP4 issues due to invalid accepted domain
8. Error when adding or removing a mailbox database copy

Read the full article here:
http://msexchangeteam.com/archive/2010/09/01/456094.aspx