Well, if we want to move forward with features and technology, relying on users who're still operating with IE5 will really hold things back...
I'm sure IE7/IE8, FireFox3, etc will be highly supported featurewise; but things way outdated really do need some sort of push to upgrade.
I know Rick is making some compatibility updates for a "low fi" presentation of things for outdated browsers (hey, I recommended something like that!) as a first start.
I also know that some people are in a position where they can't upgrade; so I'm sure if something like this worked its way into the stock product that it'd be for "members only" and that each member could disable the nag just by clicking an "ignore this suggestion" option.
I do like the idea though; a lot of people just install things and forget them. They don't take the time to upgrade what works for them; giving them a push can be helpful.