I like the idea, but I'm hoping that the web based version of Balsamiq Mockups accomplishes this exactly.
The interface is so simple and they're a great company, so I'd much rather see them become the 'wireframe sketch' standard than a google docs template!
So far, gomockingbird seems to be accomplishing this, but I feel they're missing a few of the template items that I enjoy so much in Balsamiq.
The one feature that would make gomockingbird way more useful to me is sharing editable versions of documents.
We're working on a couple of features right now - collaboration is currently on my plate. You can check out what we're working on at forums.gomockingbird.com . I also made a couple of tech blog posts about the stuff I'm learning while implementing real-time collaboration at the tech blog (techblog.gomockingbird.com) if you are interested.
This is nice, but maybe I'm too old, or whatever, but I absolutely can't wrap my head around google docs file management. It's incredibly cumbersome and painful.
I personally love gomockingbird.com
This is a great resource to share and I'm glad you did. I'll be using today in fact.