If Google Wave improves, I think it would be nice to have an announcements wave for this purpose. The feature set is much more amicable to the sort of use you're describing. It's also much easier to set up a wave than to set up a website. To illustrate, I've made a Math Announcements wave. (If you don't have a wave account, email me; I've got a bunch of invitations.)
Of course, the problem is that for an announcements page to do any good, people actually have to look at it. Presumably the idea was to bootstrap an announcements page using the MO community. I think that part still makes sense, but you need to start with something. If somebody makes a nice announcements site/wave, gets a few dozen people using it and gets some content up so that it actually looks like it's going to work, I'd be happy to link to it in the FAQ and even post a system message on MO encouraging people to check it out. I'm sure many math blogs would also link to it, but it needs some steam behind it to begin with, and I think there needs to be a person or handful of people making sure it runs smoothly.
]]>