Coincidence is a wonderful thing - one of the OpenMSG programmers e-mailed me today talking about the desire to integrate SMS into OpenACS 4.
No one is working on a general alerts package of the sort you describe. We really need something like this, and we need an architecture that allows for multiple destination types as you mention. The agents that do actual delivery (via e-mail, Jabber, SMS etc) should be built using the service contract model, right?
And packages that do alerting should also do so using the service contract model. The general alerts package, and its UI, then can be thought of as sitting in the middle, providing a way for users to connect alerts from various packages to message delivery agents of their choice.
Am I on the right track? The most challenging aspect of this might be the UI - flooding the user with choices is a good way to confuse them.
dotLRN will only provide a simple e-mail service much like OpenACS today, AFAIK, because that's the way it works now in ACES. There is that nice central alerts page you talk about, though.