[doublec]

There are plenty of forks running already. There's "litecoin", a fork that changes the proof of work algorithm to scrypt in an attempt to make GPU's less useful for mining.

There's "PPCoin", a fork that adds "Proof of Stake" alongside "Proof of Work" in an attempt to reduce the energy consumption of the mining process.

There's others as well. Some bitcoin proponents consider these "scam" coins as they have limited use other than speculation, others consider them experiments in different approaches. But they do show that forking bitcoin is fairly easy.

[TazeTSchnitzel]

Don't forget Namecoin, repurposing the Bitcoin block chain for storing DNS records!