This is the bigger than Youtube, YC-mafia, etc. trying to put the nail in the IE6 coffin. IE6 is big in enterprises, and Microsoft has incredibly strong relationships with them. Plus office.com would actually be seen as an important productivity app to support, unlike apps like Youtube.
On a different note, I think their strategy of using Silverlight to enhance the experience, and degrading gracefully to cross-platform/browser HTML+CSS+JS when it's not available is brilliant.
"I think their strategy of using Silverlight [..] is brilliant."
Brilliantly evil, perhaps. All these stupid problems moving off IE6 have been because of people using, then relying on, proprietary, non-standard additions to the browser. It seems like a good idea at the time; a few years later it's an albatross around your neck.
So here we are, a few years later - lesson learnt? Hardly. The "solution" to losing IE6's proprietary, non-standard additions to the browser is to use another proprietary, non-standard addition to the browser. Wonderful. In 6 years time they'll be going through the whole process again - finally we can dump silverlight!. Perfect for MS, of course, they couldn't care less as long as you're still using Windows.
Anyway, luckily consumer web developers did learn the lesson, which is why Silverlight will never achieve any real penetration in the consumer market.
Silverlight has an open implementation which Microsoft has assisted with. I'm not sure how far behind the official releases it lags, but it has an open implementation.
For some things, using RIA frameworks eases development massively and lets you do stuff which would take ages to do in plain HTML/JS. Google themselves said Google Wave took around 2 years to make. Do you think it would have taken that much time using non-HTML technology?
I'm not saying sticking to plain HTML/JS isn't admirable, but sometimes it's just not very practical.
Also in this case it isn't required, so your potential scenario about being "stuck" with it doesn't really apply.in any meaningful way. In fact they have done just as you said, but given people the option to use a plugin to make things better. It degrades by default, which is helluva lot better than most sites out there.
I don't see you cursing Flash in the same way as you do Silverlight in. What makes Adobe Flash any better?
On a different note, I think their strategy of using Silverlight to enhance the experience, and degrading gracefully to cross-platform/browser HTML+CSS+JS when it's not available is brilliant.