In order to get some of the more complex Plasmoids to work correctly in (the Qt 4.4-based) KDE 4.2, those Plasmoids contain workarounds for those problems, workaround that aren't necessary for Qt 4.5 any more. Some of those workaround are even considered harmful when using Qt 4.5 (i.e. they will simply break things such as screwing out the layouting of widgets). The result of that is that Plasma in KDE 4.2 is effectively incompatible with Qt 4.5, hence the reason we ask distributors to not ship KDE 4.2 with Qt 4.5. At least not until further notice.