Reading Sebas’s blog (which unfortunately doesn’t offer way to comments, which makes the only way to comment, a blog post…), sounds like KDE4.2.0 is not compatible with Qt4.5, which is hardly a surprise, but what is more annoying is that it sounds like the whole KDE4.2.x serie is going to stay incompatible with Qt4.5 (I might be wrong on that point, but Sebas mentioned KDE4.2, so it’s unclear whether it is 4.2.0 or 4.2.x, in case I took the wrong assumptions, don’t hesitate to comment on my mistake and ignore what’s bellow).
The main thing is that all applications other than plasma or KDE4.2 might need to use 4.5, because they want to use a new feature, or an important bugs was fixed in 4.5. This is especially true in KOffice where one of the core component of an Office suite is developed inside Qt: the text engine. But this might be true for other project. Blocking the upgrade of Qt for the next six monthes seems very wrong to me. I hope plasma developers will reconsider their position, and provide a way for plasma 4.2 to work nicely with Qt4.5, so that progress in other projects isn’t disturbed by this.