This could really faciliate sender-stored email (SST-Mail)!
I'm curious about the security of it, but apparently Opera operates a site that acts as a sort of gateway. The browser connects to that site and access tunnels through NAT-ty firewalls and such. This is all speculation from me, but, if that's close to how it works, that's not bad and it's a lot better than the disaster of Windows running IIS on every workstation.
Since there's a gateway, you can impose access control and specify (via Opera userids) who can access your site. (Wow, that's even better for sender-stored email!) The more I think about it the more brilliant this seems. Surely someone has done this before but, if so, it probably didn't get the kind of public visibility that this implementation will get.