just like "clouds", "waves" do not reside on your computer, but rather *out there* somewhere, that you can *probably* get access to...
What you're saying makes as much sense as saying you can't get to your mail if you're using IMAP, because IMAP mail is *out there* somewhere.
You clearly have not read any of the technical information about Wave.
Waves are comprised of "Wavelets," the pieces that make up a wave. You could absolutely write a client application that downloaded and cached everything locally, and most clients probably will. I imagine Google itself will have some HTML5 local caching (google gears, etc) to allow you to retrieve existing stuff when offline, as long as you previously synced it up.
I mean, they already have offline Gmail support. Wave is along the same lines.
Wave is absolutely going to catch on. I think in 20 years you're going to be saying the same thing about email that we're saying now about things like Gopher vs. Web.
The real news here is not that it's open sourced -- Google said that from the beginning. Everything about Wave will be completely open. The news here is that there is some code shipping.