There are more possible bitcoin addresses than grains of sand on the Earth. It's so unlikely anyone can generate an address belonging to someone else that it's virtually impossible.
People have tried generating millions of addresses to attempt finding a funded one, but they all failed.
This thread is about one of those attempts.
https://bitcointalksearch.org/topic/large-bitcoin-collider-collision-finders-pool-1573035This quote explains why it's virtually impossible to generate an address belonging to someone else.
Don't worry, "If you were to intentionally try to make a collision, it would currently take 2^126 times longer to generate a colliding bitcoin address than to generate a block.". This means, if you have a computer that is 1 million times as powerfull as all current miners combined, it will still take an average of 1,618,542,460,620,902,128,345,579,373 years to generate a collision.
Even if Moores law holds true in the most generous way, we still have over 100 years left before this becomes feasable.
And also: yes, devices designed specifically for performing bitcoin mining exist (Artforz had himself some ASICs (custom chips) made)
Read this whole thread for more information.
https://bitcointalk.org/?topic=62.0 
