Wil had the right idea, but the wrong format. Think SMS, aka text messages. Instant messages still suffer from inter operability problems, however AIM is better than nothing.
RSS is great for pushing out info, but there is a time sensitivity issue. If, using Wil's example, you had an appearance you would need to make sure to get it in your feed far enough ahead to a) allow people to see it in their feed and b) allow people to make plans. B obviously needs a bigger time lead than A, however you still would need at least 1 or 2 days to have a reasonable chance of letting everyone at least know.
However, what if you were asked to do a last minute fill in on Letterman? What if you need to cancel a meeting at the last minute with a group? Not everyone has their IM client open 24/7. Could you drop a quick text message to list and get near instantaneous notification? Not a new idea, but not one done here in the States. Mostly because US cell companies see text messaging as another way to
However the flip side to this was a somewhat recent article on 43 Folders. RSS is great, but some of its uses have left us spending too much time managing our feeds sometimes. Wouldn't it be great if there was a way for readers to detect and let you know "we see your delivery of yak hair has arrived. Do you want to remove feed foobar?" or maybe "You have not received something on the foo feed for 30 days. Remove it?" I would not want to automatically whack a feed - some of mine intentionally receive little traffic but I want that traffic when it comes.
I guess there is just a matter of how we get info (there is a timeliness issue as well, but Merlin touches on that at 43 Folders). Think about it - many of us probably have an IM client or two, an email program, maybe a couple of web email accounts checked on the web, and an aggregator (or two). This does not include multiple devices many of us have (cell, PDA, multiple computers, god knows what else) or services we use to keep some of our local data synched amongst the different machines (think NewsGator & Feedburner). For some eliminating one piece of the puzzle is a great help.