Regardless of where you backup, Time Machine is still an issue with laptops. I have clients who use their machine, then sleep it. Then wake it, take it to the couch, do work, etc. I've had to have them go on a 5 min screen saver, 10 min screen off, 2 hour sleep setting guide on power adapter only with drive plugged in just to make sure TIme Machine will get something. Remote backup to iDisk would resolve all of that, backup to iDisk while you work on your laptop on the couch. Done.