UPDATED (11/15/07, 4:20 PM PST): Activation for iPhones added, support for PPC added. There's also a fix for the SpringBoard crash issue.
http://conceitedsoftware.com/iphone/1.1.2-jailbreak.zip
Read the README first.
Notes:
Yes, you WILL lose your existing apps and must reinstall them. There's no perfect way to resolve that. You could back up your applications to your Media partition beforehand and move them back afterwards, though I would caution against replacing 1.1.2 apps with old 1.1.1 versions.
Some notes on upgrading (not really related to the jailbreak itself):
Unless you have a good reason to do so, do not restore from a backup when iTunes offers. The new version of SpringBoard evidently chokes on some of the (jailbroken) data files that would be restored this way.
Some 1.1.1 apps will not work with 1.1.2. Try to get 1.1.2 versions of all of your software.