What do I have to do to unjailbreak from Absinthe? I'm a little new to the whole iPhone / iTunes world, but I'm planning on eventually jailbreaking my iPhone 4S with Absinthe. The instructions tell me to erase all data from my phone, then restoring from a backup after jailbreaking to get my data back.

But what would I have to do in order to actuall remove the jailbreak again?

The Absinthe README doesn't mention this at all, and the website doesn't seem to have that info either.
To jailbreak with Absinthe they tell you to erase all data just because the software works faster, but you can do it with all your info. It will only take more time.

I recommend you to use the last version of Absinthe.

To remove jailbreak you have to restore your idevice. (it will restore with the last iOS version at the moment).