[285765]

I'm thinking of embedding a neodymium magnet in to the top headband of my ATH-M50's, and embed a magnet on to my wall so I can just stick my headphones against the wall.
I know there are magnets on the drivers, but would a magnet on the top of my headphones effect the sound quality or anything else with the headphones?

 

[HolyCheese]

I don't advise it. If you'd lay the headphone on your lap or somewhere close to your telephone/credit cards it might damadge them. 
As for the sq difference. I don't think it would be noticable but to be absolutely sure you could use a piece of iron for the bottom as shielding.

 

[mono]

Magnet field strength GREATLY decreases with distance, it's not going to affect sound quality.  The only real issue is what HC pointed out, they can wipe out the magnetic strip on credit /etc cards but even then you'd have to wipe it against them.
 
The main concern I'd have is how you're going to get the magnet in there and the repetitive stress of pulling it away from the wall.  You might want to reinforce that area.
 

 

[DrNickRiviera]

(stupid?) question: Why not just use a steel plate on the headband and a (stronger) magnet on the wall?