I think the situation is the same as with Powerbooks: you can use the lower-powered adapters, but the computer will not charge the battery and run at the same time, you have to sleep it to have the battery charge.
There shouldn't be any problem just running it or just charging it, and I've never heard of someone burning out their adapter like that. My friend would charge his 60w powerbook all the time on my 45w adapter while he was waiting for a replacement to come in the mail last year. Never a problem.