This is true, you need the callback=? in the url string when using $.getJSON method… but why?
Here’s what jQuery API says:
JSONP
If the URL includes the string “callback=?” (or similar, as defined by the server-side API), the request is treated as JSONP instead. See the discussion of the jsonp data type in $.ajax() for more details.
from source: http://api.jquery.com/jQuery.getJSON/
I know this is jQuery related, but I think Twitter should openly post this in their API documentation. Something along the lines of “Remember, if your are making the request cross domain make sure to request in JSONP instead of JSON” at the very least. Bonus if they add “for example when using jQuery add callback=? to the URL”.