This is a user to user forum. There is no moderator participation explaining anything, just we users are here for that.
Many things about Macs are auto-detected by them and when detected the appropriate preferences appear as if by magic. Someone with a new iMac who has successfully connected to an input source would be able to say definitively if the preferences change.
As for what can be hooked up to the port and used, most of the adapters of which you refer are uni-directional, designed for video output only. Apple states explicitly in the webpages describing the new iMacs and their features that the port accepts a signal from another device with a mini-DisplayPort only. So if you have a Mac laptop with a m-DP you can hook it to your new iMac as the monitor with a m-DP to m-DP cable. Apple sells one made by Belkin in the Apple Store.
The other thing about the m-DP is that is does not carry audio. So the idea of hooking it up to a Blu-Ray player is still just a hyped idea. However, it appears that Belkin is developing a black box to facilitate this idea. Earlier in the week some folks here in the forum linked a Belkin website page with an HDMI to m-DP adapter to convert an HDMI signal to m-DP input. Later this week Belkin took down the page, so perhaps it is not yet ready for production and sales! On one side is an HDMI input. Coming out the other side is both a m-DP and a USB port. I think that the black box separated the signal into audio, to send to the Mac over USB and video, to send to the Mac over m-DP. The USB port could also supply any power the box may need to its job.
Dah•veed