It's almost the end of June and there's no sign of KDE 4 RC 1. The last development release was Beta 1 that were released last month. Usually, they provide RC 1 to make sure there's no regression and people would have seen the (quite) stable version that are much likely similar to the final version which is planned to be released on July.
Will it be delayed again? I'm not sure about that, but i do hope they will release the RC this week, so they have enough time to get the feedback they needed to make sure it has fixed all the remaining blocker and major bugs.
So far, only Fedora and OpenSuSE 11.0 which has used KDE 4.0.x as the default window manager. Slackware team will try to test KDE 4.1 when it comes out whether it's stable enough to be included in the next Slackware release (12.2 or probably a bump to 13.0). It has big dependencies and lots of efforts, but Robby Workman has tested and provided the binary packages (and also the source) to be tested by public. You can get them in Robby's page