Well, dont know if that is on road map but multisite owners definitely need this centralized system, with possibility of sending a nice "digest" mail.
For now i use subscribe 2 (sorry guys ^^) and Subscribe to double optin comment. But those are only per site basis plugins, which allow to put them as premium plugins in pro sites, sure, but the very coooooool WP.com subscription system is just... Well, worth a lots of raccoons ^^
How do you do on edublogs ?
Any hope ?