Apple has recently released 3.1.3 with some minor bug fixes, you can read our post on the changes in 3.1.3 and here is how you can jailbreak 3.1.3 firmware on iphone 2g and other old models iphone 3g , ipod touch 1g and 2g.

Warning: We suggest you not to update your iPhone to OS 3.1.3 if your iphone is currently unlocked, jailbreaked or on firmware 3.1.2. As the new iPhone 3.1.3 firmware updates the baseband to 05.12.01 on iPhone 3GS and iPhone 3G which cannot be unlocked now. 

Update: iPhone Dev Team has released redsn0w 0.9.4 which can jailbreak iphone os 3.1.3

Here are the direct download links for 3.1.3 firmware

Link to download iPhone 3.1.3 firmware for iPhone 2G
Link to iPhone 3.1.3 firmware for iPhone 3G
Link to iPhone 3.1.3 firmware for iPhone 3GS