Depends on whether you've already got Qt4.5 installed or not. If so, there should be little change (as that's now being removed), otherwise it'll be quite a chunk extra needed.
Some welcome news for all Qt on Maemo developers out there: Nokia intends to deploy the currently in Beta Qt 4.6 to all devices via software update — most probably already with the next major update PR1.2. Currently, Qt 4.6 (available from extras-devel) is completely optified and resides in /opt/qt4-maemo5. That will change with PR1.2, where it will move to the rootfs, replacing the community-supported Qt 4.5. What does this mean for Qt applications? Qt 4.6 applications: If all goes well, nothing needs to be done. Deployed applications should pick up the libraries in /usr. You might need to edit your build scripts so they won’t look in /opt/qt4-maemo5/bin if you’re using qmake. Qt 4.5 applications: This one is trickier. Qt 4.6 is neither API nor ABI compatible with Qt 4.5. This means that you’ll need at least a recompile, and probably also source code modifications if you’re doing anything Maemo5-specific. Qt 4.5 applications on device will stop working until there is a Qt 4.6-based update available. So all you Qt 4.5 users out there, install libqt4-maemo5-dev in scratchbox and start porting.