Any ideas on how to jailbreak and unlock an iPhone 3GS after installing the new 4.0 software?

You may not be able to yet. Have you heard whether or not someone already provided the software?

If so, I would assume just download it, and jailbreak it the same way you did before it was upgraded to 4.0

http://www.redmondpie.com/jailbreak-ios-4.0-with-redsn0w-9170213/

This is the proper link. You can download Pwnage Tools here. Currently, the only 3GS models that can be jailbroken are the ones with the older boot rom. 359.3 is the old boot rom, which can be jailbroken. 359.3.2 is the new boot rom, which cannot be jailbroken at this time.

You can jailbreak your Iphone3gs in this site http://jailbreakiphone3gsing.com/ . After jailbreaking you can unlock iPhone 3Gs from network lock using remote unlock service.You can get remote unlock service for your Iphone3GS in this site http://www.mobile-unlocker.com/apple-iphone-3gs/rs16wp9/.