It doesn't need to be an official exchange rate, it's what I guess would be best translated as a "fair market value". If there appears to be no fair market value for the virtual currency, one would need to be agreed upon by the parties. (I base this on the accounting rules listed for the receiver of the virtual currency).

What I find curious is the asymmetric handling of realized exchange rate profits vs. losses. Profits are taxed as capital gains, but unlike with e.g. shares, you can't use realized losses to offset the profits.