It is my understanding that assets (in this case bitcoin) are not taxable until CONVERTED to dollars by sale or other means. Also, you can claim self employment and depreciate the machines used to mine. Not full cost, but over time based on their useful lifetime. If you go that route, you WILL need a CPA to check your figures and sign off on them, for the inevitable audit.
Here is my opinion. If you are a sole prop LLC (which you should be)
Your hardware to produce Bitcoins/other coins is an asset like a printer, a server or a laptop. You use it to work and make money.
You purchase it and write it off over a reasonable depreciation period (usually 3 to 5 years)
Bitcoins that are not converted to cash have no value and are the same value to your LLC as a domain name (something you could sell and convert to cash) but if you haven't you have no realized value.
If you were wise enough to register the domain name "creditcards.com" that obviously is worth hundreds of thousands of dollars, but until you sell it for cash it is worth the $8 your paid GoDaddy to register it.
Same for a string of characters that is a Bitcoin, until someone buys it from you, it isn't worth squat.
So if you paid $20k for miners and never sold a bitcoin in 2013 you have $20k in depreciable computer assets and nothing more.
Your mileage may vary, batteries not included, items in the mirror are closer than they appear, CONSULT YOUR ACCOUNTANT.
Don't mention the word Bitcoin in your tax return.
If you feel compelled then they are:
Application Specific Computer Systems.
Good luck