Go ahead push it out...
Personally I think they should push it out, similarly to how they pushed out IE8. Let windows update download it, but durnig install offer up a box asking if the user wants to install it.
Those that already have protection can just say no.
Those that don't and hence putting the rest of us at risk (and our mailboxes) probably aren't going to go hunting for some security software themselves, even if it is from Microsoft, but give them a big window with 'click here for some free protection' and they just might do it.
Okay that's the serious bit, now onto a minor, but real ulterior motive....
if a side effect is that Symantec and McAffee die a horrible, agonizing death, then all the better :-)