The delay, mentioned in this mailing list post, is so that Qt 5.0 can be of "quality to a very high standard." The main items to still work out are to produce Qt 5.0 binary packages and installers, finishing the documentation modularization, further improving the platform plug-ins on Windows/MacOSX/Linux.
In terms of actual bugs for Qt 5.0, he believes they are in good shape. But, "To accommodate for that I propose that we now rather accept a delay in our release schedules and work towards a new timeline that I believe we can meet. I'd like to aim for having the first beta (with a hard API freeze) our by the contributor summit. Depending on the feedback we will then add a second beta beginning of August and aim for a final release sometime mid to end of August. In the meantime we can/should think about doing another alpha. Thiago proposed that one option here could be to simply release the first source package that works on all platforms as the second alpha. Like that we don't use any additional resources and time on creating that second alpha."