Regarding AnonyMint's concern regarding concentration of mining power, something hit me this morning:

What the IRS issued recently was guidance.  It was their view of how the existing laws should be interpreted, given their knowledge of bitcoin.  However, the courts have the final say in the interpretation of law, and by making rulings they set precedent moving forward.  

From my discussions with various community members, I think the general feeling is that hashers are paid for the services they provide to a mining pool whereas miners create bitcoins based on their own initiatives and the acceptance of their efforts by their peers.  I think if the IRS had a deeper understanding of bitcoin, they would have used the word "hashers" rather than "miners" in their guidance.  

Based on existing laws, I believe miners (but not hashers) are free to recognize gains on any coins they create when a gain is realized, and a court challenge would rule in their favour.  But understand what this would mean: it would mean that mining on P2P pool imposes no reporting requirements on you, whereas mining at a pool like GHash.io does.    

It would mean there is a legal incentive to decentralize.  

This is my opinion and should not be construed as legal advice.  IANAL.