Gains are taxable in the year they are realized. Realization occurs when you exchange bitcoins for any type of other property; such as cash, merchandise, or services. This includes everything from haircuts to yachts. Essentially, any transaction involving Bitcoin is a realization event and triggers taxable gain.
Because I’ve seen a lot of misinformation on this point, I want to make myself perfectly clear. If you own bitcoins that have appreciated in value, you cannot use them to purchase goods or services without realizing gain. Such a purchase is an accession to wealth. It puts you in the same position as if you had first sold the bitcoins for cash and then used the proceeds to purchase the goods or services directly. Yet, one would be a taxable transaction while the other would not? The IRS would never tolerate such a blatant loophole, and neither would the courts. In fact, this exact argument has already been rejected for other types of assets. The outcome for bitcoins will be the same.