From the Apple web site:

The iPhone 2.1 software update contains many bug fixes and improvements. To get it, connect your iPhone to your computer using iTunes 8 and click Check for Update.

Update includes:

• Decrease in call set-up failures and dropped calls
• Significantly better battery life for most users
• Dramatically reduced time to backup to iTunes
• Improved email reliability, notably fetching email from POP and Exchange accounts
• Faster installation of 3rd party applications
• Fixed bugs causing hangs and crashes for users with lots of third party applications

• Improved performance in text messaging
• Faster loading and searching of contacts
• Improved accuracy of the 3G signal strength display
• Repeat alert up to two additional times for incoming text messages
• Option to wipe data after ten failed passcode attempts
• Genius playlist creation