It seems to me there is a real risk in Trolltech's going public that another larger company with objectives that are not really compatible with open source could buy a controlling stake in Trolltech and then be in a position to implement a number of closed-source strategies which might include making Qt closed-source, making the Qt development tools closed source, or even ending all Qt development.
I belive there are legal agreements with some of the KDE folk that prevent this happening.
The worst that could happen would be for future versions to be closed source but the opensource community could continue to build on the current versions.