It is not GWT. It is very similar to the other JS-heavy Google Apps (GMail, Docs, Buzz, etc.) I'm not sure how many details of those are public, so I'll leave it at that.

Edit: Actually, apparently parts of the stack have been open-sourced:

http://code.google.com/closure/

http://code.google.com/p/protobuf/

http://code.google.com/p/google-guice/

How do you know?

I had a very tangential role in getting Google+ out the door (as in, I got pulled in for last-minute bugfixing and fixed a handful of bugs), and I've been working in the Google+ codebase for the last few weeks.

http://ssl.gstatic.com/gb/js/pwm_377e54339fe6909b01c5d023b52... uses goog.structs.SimplePool, so you know at least that part uses Closure, http://closure-library.googlecode.com/svn/docs/class_goog_st....

I'm pretty sure nostrademons works for google...

At least for Google Closure they show where it is used:

http://code.google.com/closure/library/

Not sure about the other libs...

For a list of the Google products that use GWT, there is this recent discussion in the GWT Google Group:

http://groups.google.com/group/google-web-toolkit/browse_thr...

Well they seem to have borrowed something from Orkut: …notifiert-hangingget-js.js var bf = "orkutFrame";

I assume since no-one has mentioned Google+ in connection with Google App Engine, that is not built on top of it.

Sure Google+ probably shares infrastructure with GAE, but you would think that if Google were really serious about GAE they would use it for their own external products. It sends a mixed message to the market when they tell app developers to use a technology for mission critical apps that they are not using themselves.

I would be curious to know how much (and which parts) are Go powered as well.

See http://gigaom.com/video/google-hangouts-technology/ for details on Hangouts (video chat).

Oh Cmon, we all know it's PHP and MySQL :P

Unfortunately, no one can be /told/ what The Google Stack is. You have to see it for yourself.

"Look, maybe you just suck at explaining." http://xkcd.com/566/

I would be thoroughly impressed if any of it is supported by Go.

Does anybody if and how developers can interact with G+? Is OpenSocial going to be included?

I'm going to guess that it is largely written in Java and uses google's typical stack of technologies (BigTable, GWT, etc), but I'm not sure if any google engineers have let anyone know yet.