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.
* 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.Security fixes
iPhone Software 2.1 also patch six security vulnerabilities.
* An application may be able to read another application’s files.
* Multiple vulnerabilities in FreeType v2.3.5.
* mDNSResponder is susceptible to DNS cache poisoning and may return forged information.
* Predictable TCP initial sequence numbers generation may lead to TCP spoofing or session hijacking.
* An unauthorized user may bypass the Passcode Lock and launch iPhone applications.
* Visiting a maliciously crafted website may lead to an unexpected application termination or arbitrary code execution.Other fixes
Some of the other enhancements and observations reported by readers thus far:
* No sign of background push notification or turn-by-turn GPS support.
* The texting memory leak problem seems to be fixed.
* Apps are running a lot faster.
* No scrolling lag, faster launching of SMS app.
* New “3G” logo in the menu bar sans box.
* Updating applications keeps their icons in place on the home screen.
* App syncs still sluggish compared to media syncs.