I agree with the above couple of posts. I'm at a big 4 in tax, and about the time you're an experienced senior associate (3-4 years in), opportunities start popping up for rotations abroad. These rotations are generally between 3 months and a year, but occasionally, they can become permanent under circumstances particular to that person (I'm sure the visa situation is key).
However, these opportunities are usually limited to their usefulness to the firm. Like, if you wanted to do a rotation in Hong Kong just because you like Hong Kong, the firm wouldn't go for it unless they specifically needed someone in your practice with your skill set to go out there. The most common for my firm is the US tax desk in London, because of the economic relationships, the common language, the size of the office there, etc.
It's also worth noting that these are competitive positions and people in the tax practice all over the country can apply. My impression has been, though, that if you're a good worker and you can get good recommendations from your managers, partners, etc., then it should happen (but maybe not to your first choice).
REG - 81
FAR - April 13th (73), TBD
AUD - July 6th
BEC - TBD