If you are writing off your mining equipment as a tax loss, I'm sure you are also paying taxes on Bitcoin earned as either income or capital gains, yes?
If you treat BTC as commodity money like
this federal judge does, then a taxable event will only occur when you convert your BTC to USD (or when IRS takes BTC directly as tax payment, which requires legal tender status) , and yes the entire conversion is now taxable as capital gains since you have no tax basis to start with (e.g. you did not buy the BTC with USD initially, you mine it with the rig which has already been written off)
First you need to establish that your activities in Bitcoin Mining constitute a "business" and not a "hobby".
If it is determined that your activities are a "hobby", then you'll only be able to deduct the expense from the revenue you get from the hobby and not from any other revenue you may have.
If it is determined that your activities are a business, then you may be able to deduct a larger portion of the expense, however, you may be obligated to deduct the depreciation over multiple years rather than taking the full deduction in a single year.
Note that while you may not be required to pay income taxes on the inventory you hold (this isn't entirely clear yet), you will probably be required to pay taxes on the bitcoin you spend, even if you don't convert it to fiat first.
