iOS 4 is laggy for most people who have the iPhone 3G. I have since downgraded back to 3.1.3. It’s much more usable even without all the new features. So if you upgraded to iOS 4, I highly recommend you downgrade.

Please follow the instructions here:

http://www.iphonehacks.com/2010/07/how-to-downgrade-iphone-3g-from-ios-4-to-iphone-os-3-1-3.html

When syncing a device using iOS 3 or later, syncing may stop with Error 13019.

Learn how to find out which version of software is installed on your iPhone, iPad, and iPod.

You may encounter the following issues:

You are unable to receive MMS messages
You do not see the camera icon next to the message field:

A red exclamation mark appears next to MMS messages you send:

Note: MMS is not available for original iPhone

It is important to create frequent backups of important data. This is even more important if you are using beta software. You should make sure you back up your data prior to entering the MobileMe Calendar Beta.