[rayiner]

That's more or less the rule: the gain is taxed when it is "realized" which is roughly when it becomes "yours." It doesn't actually have to be cash in your hand (otherwise it would be quite easy to get around tax laws simply by trading assets on accounts without taking cash out). See: http://en.wikipedia.org/wiki/Realization_%28tax%29.

[arethuza]

That's pretty much the same as the UK (and presumably everywhere else).

Pretty ghastly situation to be in if you do end up with a tax liability because of something like this.