I stand corrected. But that is so far in the future as to be a non-issue. In the short term, we'll use smaller denominations. Moving the decimal would require a re-write of the client, and would likely cause a fork, as above.
Yes, it is very far off, but I wouldn't consider it an issue, but a solution. Many people are afraid Bitcoins will become obsolete when they hit 21 million, and that people will not want to pay 0.00000015 for a drink or some such thing. It makes it more aesthetically pleasing to move the decimal point over.