While I like the idea of running a VM in a browser (not sure if I'm convinced it makes sense, I still like the idea):

Argument #1 only makes sense if you support a lot of platforms. Right now you only support Mac OS X (according to another comment).

The number of people who use Chrome globally is larger than the number of people who use Mac OS X. Ergo, if you used Native Client and chrome, you'd be more ubiquitous.

Regarding sockets: chrome supports UDP and TCP: http://developer.chrome.com/apps/socket.html

If there's one platform to build on that is going to cover a large number of people and give close to native performance, it's Chrome+PNaCl. I wouldn't tell everybody to drop what they're doing and adopt that target (it's not ready yet). VMs in the browser are pretty nascent, too.

Why do you think that is? Is it because Google have a large team of engineers who have developed a proper security programme for the project and realised that doing 2 and 3 are bad and that as a result of that and other problems with diong this that 4 is necessary? There are reasons for these limitations.