I have what look like memory leaks (and which may or may not be genuine memory leaks) with KDE, whether I use Konqueror, or not (KDE 4, of course). I haven't checked with any other GUI recently, so I can't offer a comparison with, say XFCE or Gnome.
I end up re-booting every three or four weeks, and I may even get to a re-install before my next 'scheduled' re-boot, so I was thinking of investigating a bit more if the problem doesn't get better then, with a more up-to-date version of KDE (currently at 4.6.5 - I was rolling kde, at one point; it not currently that bad, and I hope that there have been more improvements subsequent to 4.6.5). But I'm certainly interested, because rebooting every few weeks is a bit of a pain.
My memory is that this did not happen back in the KDE 3 days, and while KDE 4 has brought a number of desirable things, stability still isn't back to where it once was.