That's what I meant - most probably some server support will be required. I hope ejabberd and others will address this. Until that WebRTC won't be useful for building web based XMPP/Jingle clients.
Regarding codecs - most XMPP/Jingle clients support VP8 and Opus is catching up too. It's not like there are many of them around anyway. Farstream supports both if corresponding gstreamer components are present.
IIRC, ejabberd is agnostic about the XMPP messages going through, you should be able to use strophe.js to craft the proper Jingle packets from the browser's offer/answer session description.
Given codec and SRTP compatibility with clients, eg GTalk (which I don't know about), you should be good to go.
But ejabberd still had to enable support for BOSH or WebSockets first. Without it Strophe was of no use. So I assumed that similar thing has to happen with in this case in order to be able to route the Jingle stream from the browser over WebRTC. If the server modifications won't be needed at all - then of course it'll simplify things. Also, clients don't always use SRTP - there can be other methods like ZRTP for example.
Regarding codecs - most XMPP/Jingle clients support VP8 and Opus is catching up too. It's not like there are many of them around anyway. Farstream supports both if corresponding gstreamer components are present.