They haven't really been done to the core of IE before, have they ...
Sure, the "core of IE" has been done with the same add-on technology exposed to 3rd parties.
Favorites and History are Explorer bars, same technology as 3rd party explorer bars.BR>
Autosearch from the address bar uses IURLSearchHook, documented since it was implemented. In fact, that's a weakness of the current implementation. The GUID for the default SHDOCVW implementation is right there and easily stomped by malware.
... so is this an indication that there will be a pre-Longhorn IE addon with all the new features?
Gary Schare's interview on BetaNews said talked about the same stuff. But, as always, we're not going to make empty promises so the lip is zipped right now.