It started as a bookmarklet, but the endpoint URL for the mobile share form is easily extracted from there (note: this works, ok but is not ideal… see Update #2 below):
We’ve gone ahead and implemented Google+ now, so if you are a fan… happy linking!
Update #1: In playing around with it a bit more, there’s a strange behavior whereby Google says there’s a problem with the post, but it actually does get into your Stream. I’ll dig around a bit and see if I can sort that out.
Update #2: Google is finally supporting this properly. Here’s the URL scheme: