For people who want to play at whatever the D&D world has for cons or even drop in game at random stores, etc, it's probably more of a problem since if they are all playing 4.0 you need to too.
Good point. For those that want to play in the competitions, they are probably out of luck, if they want to continue playing older versions. If they want to play at cons or gaming stores, I bet they can still find older version games to play in, though I'm sure they're rarer. And if they offer to DM a game, I'm sure they'll find players. There are plenty of people who won't switch, or will only switch grudgingly, who would be happy to play an older version.
And I still use Office 2000.