The jury's still out on Windows Live Search Cashback -- apparently there were some issues on Black Friday and, overall, things aren't up to goal ... but that could change.
In light of this cashback program, though, my proposal from last year that Microsoft simply pay people to install Silverlight seems astonishingly realistic.
The problem with Silverlight is that it doesn't have wide enough deployment -- so there is a chicken-and-egg problem deploying apps on the platform. Microsoft has released stats about how many people 'have access to a PC running Silverlight' -- but that's an odd definition of penetration (if a state university has 15,000 students, and 10 computers in a library lab have Silverlight, then presumably all 15,000 'have access to a PC with Silverlight'.)
So ... why not just pay folks a small amount to install? Maybe a $2 coupon(Starbucks/Amazon/PayPal/etc.) to install the plug-in in the user's default browser.
The WGA or XP key-checking components could be used to keep track of machines that have participated, making it impractical to game the system by performing extra installs. I'm sure a suitable equivalent could be used to verify installs on Macs and Linux boxes.
Another approach is to wait for a future Vista SP or Windows 7 upgrade cycle. Since Silverlight will presumably be included for IE, the trick is to hook the default browser selection and attempt to install a suitable version of Silverlight into the target browser (Firefox, Chrome) when the user selects it.
This move would surely be controversial -- even if there were a way to opt out -- and would bring back memories of the Microsoft of the 90s for some. But, since the web page itself decides what plug-ins to use, and no content is "forced" to render via Silverlight, it's only moderately different from making WMP or any other dynamically loadable object available.