Aight, ATM's have been placed in SL by Rosa and Blad, and then the main InWorldz one is on InWorldz Desert Island.
Just a quick walk through for you guys, although Blad already got to play with it today

If you want to transfer L's to I'z, find an ATM in Second Life. If you want to transfer I'z to L's, find one in InWorldz.
When you find the ATM in the world you are in, simply pay it the amount you wish to transfer. All transfers are done at a 2:1 ratio, so if you transfer 300 L's, you'll have 600I'z here and vice versa.
One you have paid the machine, log into the other world, and find the ATM there, when you get to it, type in
withdraw in the chat console. That's it. You'll then have your money.
Now, I do ask you please, if you are transferring large quantities into Second Life, give us 24-48 hours to complete your request. A simple email to let us know ahead of time would be great, although we do get an email every time a transaction is processed.
Enjoy!