Actually I think the master plan is to switch GMail to use Google Wave (note that the demo looks almost exactly like GMail- except better). Suddenly you have a bazillion Wave users. Developers would love to target all these users. Developers start generating lots of different kinds of waves. Email is no longer lame- you can create a custom wave robot to augment your email client however you please. An Mp3 wave, a document sharing wave, a Go game wave, a Hacker News wave, a realtime peer coding wave... Hopefully this will inspire the open source community to set up non Google hosted wave servers and services.

Google Wave also seems like it might be a solid scalable technology on which to build P2P streams.

I work on an open source meta-web project called ShiftSpace. The Google Wave server looks like the perfect backend for what we're working on.