Users have to login to each domain separately

I have the Domain Mapping plugin installed however when a user logs into one domain they are not logged into the other domains mapped with this plugin. Currently users have to login to each site individually.

The plugin's description says:

And, naturally, we've built in cross domain cookie syncing, so your users will stay logged in (or out) regardless of whether they are on your standard domain or their custom one.

It seems to me that users should only have to login once. Am I reading this wrong?

I searched the forum and found several threads that say that the plugin author is working on this but I haven't seen any updates since version 0.3 has been released.