Google recently opened their Wave server for federation, meaning that anyone can download and deploy a Wave server.
How much 'server' is required to run Wave for a given number of active users?
I understand this is a difficult estimate to make, but I'm hoping that someone will have some metrics. If not now, then soon.
Java has increased in speed and performance, and I'm certain Google has been coding for speed, so I'm hoping that one can run a small wave server on a VPS, and a reasonably sized Wave community would only consume a single dedicated server.
But currently email servers can handle tens of thousands of users per mid-range server, and I'm worried that wave will require more than a single order of magnitude greater resources.
You say that email server can handle tens of thousands of users per mid range server, yet this is not strictly true. Sure a simple postfix server with POP 3 access can handle this, but all it does is email. If you want to bolt on things like calendaring, contacts, shared documents etc you've suddenly got an Exchange server and it can hold substantially less users per server.
Its the same thing with Wave, you're taking email and bolting lots of other things to it, so sure it's not going to be able to support tens of thousands of users per server, but you can do a lot more with it. So yes, I would imagine it's going to need more resources than a simple email server. Java may have got faster, but it's still pretty hefty so your Wave server's never going to be able to hold those tens of thousands of users, probably substantially less, but then if you want the features it'll probably be worth it.
I'd suspect a small wave server would run on a reasonably speced VPS, it'll probably be quite memory hungry, so be sure to give it plenty. Unfortunately Google haven't released a lot of information so it's going to be a case of try it and see.