There are several reports that updating to 2.0.1 on an iPhone in Airplane mode may result in bricked phones.
Users who paid Vodaphone to unlock their phones for international roaming have reported the update breaks connections with their carriers.
A few users report occasional lag in the Contacts, SMS and e-mail apps. From our testing, typing in the Contacts app is still laggy, but SMS and e-mail is faster.
No surprise whatsoever: Updating results in losing access to your jailbroken (i.e. hacked) apps. 2.0.1 updates iPhone 3G's baseband, you may never be able to unlock the handset again if you run this update.
Many users are reporting 2.0.1 is slow with backup. This more likely has to do with the version of iTunes they're running; many have said iTunes 7.7.1 sped up backup and syncing.