Actually, that is not necessarily the case. Depending on your RAID setup, many of the advanced RAID cards support dynamic migration and expansion of arrays. Assuming you are using fakeraid (eg onboard raid or a cheap raid card), the answer is exactly as rofl says.
O rly? I haven't heard of a card capable of rebuilding a RAID-0 array as there isn't any data to base the array rebuilding on.
Other RAID Modes (5 and 6) do have parity data to rebuild the data. RAID 1 could simply copy over the data. What would RAID-0 do, or how would the controller determine which bits would go to the newly added drive?