It may seem reactionary, but I suspect the debate about whether to end support for IE6 has been going on for quite some time.
I would be very interested to know what percentage of Google Docs users are still using IE6. I would also be very interested in knowing how much it costs Google to support IE6 (the recent attacks notwithstanding). The development costs of supporting IE6 in any sort of web application are not insignificant. I suspect the percentage of the development budget that Google uses to support IE6 is disproportionate to the percentage of IE6 users.
Dropping IE6 is inevitable. Sure it leaves some people behind, but I'll bet the benefits of doing so significantly outweigh the drawbacks.