It would appear that way, however, when I looked at the QT-project page for supported platforms, it specifically mentions Lion as supported. It does require Xcode to be installed. It would appear that the MacPorts collection is a version behind, as QT5.0 is available on their download page.
In summary, it appears you cannot install QT on your system with MacPorts at this time, but it is available for Mac OS X Lion installation from the QT's project site.