I have now tried beta for both ubuntu and kubuntu of 8.04. Just did a fresh install few days ago for ubuntu 8.04 beta that confirms my understanding that kubuntu build isn't the same as ubuntu. I prefer to stick with ubuntu. According to this final release is expected 3 weeks from now.
If you are about to try beta I suggest a fresh install over upgrade from 7.10. You won't be getting the same desktop look and fresh install is better.