From Konqueror's side it has always been possible to embed other render engines because of the KPart concept.
Two KDE developers have been working on a Qt based version of Gecko and I think it somewhat came to a halt when only one of the got permissions to work in Mozilla's source repository and now they are busy with other things.
Personally I see little need for that anyway, besides the technical coolness of course
I use KHTML without any problems day in, day out and if I eventually encounter a site that doesn't work and I really think I want their content, I can always start Firefox for them.
Currently the only site having that privilege is our company's intranet.
Cheers,
_