Why does Google prepend while(1); to their JSON responses?

Why does Google prepend while(1); to their (private) JSON responses? For example, here's a response while turning a calendar on and off in Google Calendar: while (1); [ ['u', [ ['smsSentFlag', 'false'], ['hideInvitations', 'false'], ['remindOnRespondedEventsOnly', 'true'], ['hideInvitations_remindOnRespondedEventsOnly', 'false_true'], ['Calendar ID stripped for privacy', 'false'], ['smsVerifiedFla

[fcicq]

see also http://blog.64p.org/entry/20111125/1322185155

[topaz2]

csrf token alternative. "anti-CSRF tokens mess with caching, and require some amount of cryptographic evaluation server-side. At Google scale, that would require a lot of CPU. This sort of offloads it to the client."

[yuku_t]

Ajaxだとテキストとして取得できるがscriptタグだと即実行してしまうことを利用して、他のサイトがJSON APIを不正に利用できなくしている