It's not limited to a small number of Gmail accounts. If you use some additional search parameters you'll see a ton of compose URLs showing up in the SERPs

Searching:

site:mail.google.com gmail inurl:?to=

gave me nearly 25,000 results. Some have subject lines too.

EDIT: Here are the first 100 URLs for the search above: http://pastebin.com/X1yTW7Pr