As reported on the Remember the Milk blog:
Anything that you do offline will be synchronized when you come back online. You can move seamlessly between online and offline modes — RTM will automagically detect when you don’t have an Internet connection, and will have your tasks ready for you. If you’re expecting to go offline (for instance, those fun-filled 14 hours flying from Sydney to San Francisco), you can also manually switch into offline mode. Then, when you’re bored of the repeating in-flight movies, you can pull out RTM and methodically tag and locate all of your tasks.