When i take a break from a game, and later i come back, i often feel like i'm kind of better than i was before. So i think adding a decay to ELO isn't necessary.
Surely sports and professions require to work hard and train hard to remain at the top, but i don't think games do. Especially when someone might drop ZK for a month, but will play something similiar like Supreme Commander, and he will still be in good shape in RTS games.
Skasi
As far as i know yes, you are dissapearing from ladders after a month of inactivity. 2 months ago (i think) Godde dissapeared from ladders, and after he came back he was #1 again so it's all true what you said.
Edit: Elo decay is also avoidable, simply by playing one game once a period of time required for someone's elo to start decaying and it misses whole point anyways.