Example 3 on Page 19 seems like the most relevant tax information... talks about a barter club giving people credits to use.
You must include in your income the value of the credit units that are added to your account, even though you may not actually receive goods or services from other members until a later tax year.
http://www.irs.gov/pub/irs-pdf/p525.pdfSo, you should record taxable income when you mine or receive bitcoins for any purpose at the value of the bitcoins at that time. Then, at the time you sell or use said bitcoins, and they have increased or decreased in value, you would record a capital gain/loss in the extended amount of the difference between the value of the bitcoins at time of receipt, and the fair market value of the goods/services purchased with the same number of bitcoins.