Qt 4.5.1 from kubuntu-experimental breaks Plasma compatibility with KDE 4.2.x - it's visible for example in RSSNow, so it's not a good proposition for SRU. I've get some information from upstream that Qt 4.5.1 has some fixes but it has few known regressions since 4.5.0 but it's been fixed in next version (shipped with KDE 4.3). KDE team does not recommend or support mixing Qt 4.5.x with KDE 4.2.x so the fixes should be imported very carefully.
Qt 4.5.1 from kubuntu- experimental breaks Plasma compatibility with KDE 4.2.x - it's visible for example in RSSNow, so it's not a good proposition for SRU. I've get some information from upstream that Qt 4.5.1 has some fixes but it has few known regressions since 4.5.0 but it's been fixed in next version (shipped with KDE 4.3). KDE team does not recommend or support mixing Qt 4.5.x with KDE 4.2.x so the fixes should be imported very carefully.