Add this hosts file entry into your system to allow Cydia to cache iTunes auth mechanism. This will enable you to downgrade your firmware in the future. more info
74.208.105.171 gs.apple.com
Currently only Jailbreak is possible on firmware 3.1 the update also upgrades your baseband which is the phonecall and modem component of your iPhone. This component is not downgradable or unlockable at present so unlocking is not possible with 3.1. If you need unlock features then don’t update until a hack is found for the baseband or at least until someone can stop the baseband upgrade happening when updating to 3.1.
If you are unsure of what the baseband is think of the iPhone OS as your computer and the baseband as your ADSL modem. Contained within the 3.1 update is an update to both the OS and the modem.
Currently for Mac users you can use the PWNAGE Tool to jailbreak on 3.1 more info
There will be more tools released in the coming days no doubt
JB Applications
The following are the common jailbreak applications that I use
SBSettings
WinterBoard
MxTube
Flashlight
Five Icon Dock
iRealSMS
Lockinfo
20 Second Lockscreen (works well in combination
with LockInfo)