I blinked when I tried the live demo of SandRibbon for Silverlight, from divelements. It looks remarkably like Office 2007:

The control is in beta, and promises:

All the commonly-used functionality of the Office 2007 UI is made available for you to use with this product, and most of the less common functionality too. Customers who have used SandRibbon for WPF will find the API familiar. The visual constructs used are compatible with all other Silverlight controls, both built-in and third-party.

The company already has a sandcontrol control for WPF. However, the Silverlight control shows how well you can replicate the look and feel of a desktop application in a cross-platform browser application. Now, put this together with the automatic online/offline synch in Live Mesh, and you could have a version of Office with seamless online and offline support. Microsoft may deliver something like this in the web versions of Office 14, though it is going to have one eye on its lucrative desktop sales and I doubt whether it will really exploit what is now possible.