I use Subscribe to Comments on all my wordpress blogs, even before Tante had askes so nicely for it…
The plugin broke with WP 2.2, if you try to change you subscriptions, you ended up with the following message: “You may not access this page without a valid key.”.
So please all your WordPress users who updated to 2.2 (you really should do this!), get the latest version of Subscribe to Comments here for satisfied readers and subscribers 