Found some interesting information at: https://bitcoin.tax/faq
How are the IRS treating Bitcoins?
The IRS has recently given guidance on the specific treatment of Bitcoins and other crypto-currencies, which has helped clarified the situation.
Bitcoin is a personal property, not a currency, and so is taxed as a capital asset
Gains made from converting Bitcoins into a fiat currency are subject to capital gains tax
Purchases of goods or services with Bitcoins must also account for gains
Bitcoins and other alt-coins obtained from mining is recognised income immediately at their fair value
Mining equipment can still be deducted as a legitimate business expense
Classification
The IRS are treating Bitcoin as property. Any gains made from the sale of personal property are subject to capital gains tax.
Basically, you pay taxes when you convert back to dollars in the USA