It's simply new is all. Once you get used to it, it makes a lot of sense, and allows applications to be complete segregated and sectioned off from the rest of the code. Add a folder and you're done. Delete a folder and files for the app are gone. A lot cleaner than having to upload your files to 10 different folders (and then trying to remember what to remove).
userInfoPane is in skin_global. You can include one skin template directly from another skin template now, so you won't see this called in the source files pretty much at all. Instead you'll see this in skin templates {parse include="userInfoPane" group="global" params="$params"}
Which means the skin template will output the results from skin_global -> userInfoPane (passing $params to it) wherever you see that tag.
A lot of this is covered in articles on the resource site.
Loading a member: http://resources.invisionpower.com/index.php?appcomponent=cms&module=articles&article=7533
Outputting content I don't think I've written an article on, though I'm sure we can do so.
Extending group configuration: http://resources.invisionpower.com/index.php?appcomponent=cms&module=articles&article=7529
TONS of various information: http://resources.invisionpower.com/index.php?appcomponent=cms&module=articles&article=7536
That article is primarily related to upgrading an application, yes. However, it goes into detail about the file structure, what files can do what, and so on. It's very useful so long as you are familiar with how to do things in 2.3.
We said we were shooting for a June release, and to my knowledge we still are. We expect an RC2 and at this time that is the only further RC release I am aware of in the works.
The thing was, we were shooting for December originally. But then we had so much great feedback with the preview site we were determined not to ignore it simply for the sake of rushing the release out. We wanted to incorporate as much as possible that made sense. The spam controls were greatly enhanced. Facebook Connect was added. The personal conversations changed a lot. Friendly URLs changed a lot and were made extensible. And we won't talk about the skin changes. A ton was changed due to feedback we received, and while it has delayed our original release plans, I think in the long run everyone will agree that IPB as a product is better because of the changes.
The subscription manager is nearing completion from my understanding. It will definitely be out for testing before IPB3 goes final.