The current situation is that you have Qt 4.7 in Maemo and MeeGo, still Qt 4.6 in Symbian and you also have a recent Nokia announcement saying that the company strategy is to focus on Qt, Qt Quick and HTML 5. Qt Quick is precisely the main difference between Qt 4.6 and 4.7. Once Symbian integrates Qt 4.7 with the corresponding wider support of Qt Mobility the cross-compatibility promise will be fundamentally there. Looking at the Qt roadmap it looks like Qt 4.8 (expected next year) will bring the full materialization of this promise.