Yahoo! Feed Alerts is now available, according to RusellBeattie.com. The service alerts you when a new post is added to a feed. I would have been more excited about this about six months ago. But it's still cool.
From an association standpoint, we are all battling with the Blackberry crowd. ("Who cares if it's on the Web site...I'm not at my desk.") I do see some applications. You could have a Breaking News or Very Important News section or something - saving alerts in this section for only when immediate responses are needed. (Advocacy initiatives and crucial votes come to mind.)
I was actually more excited by a nugget he threw in at the end of the post. Phonifier.com has a basic transcoding service you can use and code you can download to make your Web site into a mobile-friendlier site. Something we all probably need to be looking into quickly, with more phones having Web capabilities and members slowly starting to play with the features.