There is a full list of changes and fixes in the new iPhone 2.1 firware on Apples site, which you can read here:

http://www.apple.com/iphone/softwareupdate/

I've also included them below:

General Updates

* 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.