(This is similar but doesn't address the fire hazard issue.)
I have a MacBook Pro with an 85W power adapter. My wife has a MacBook with a 60W power adapter.
We charge both computers with both adapters. Of course the MacBook Pro charges more slowly from the 60W adapter, but otherwise it's fine.
However, according to this comment, using the 60W to charge the MacBook Pro is a fire hazard!
Is this true? I am surprised Apple engineers would have made them interchangeable if this is the case.