Luckily the LGPL license can never be revoked on previously released versions. So it's possible to use Qt 5 and never have to worry (as long as you follow the requirements of the license in terms of dynamically linking the libraries).
That’s true for 6 as well. I believe there are clauses in Qt’s ownership documents that prevent it from ever becoming closed source without automatically invalidating previous licenses and becoming GPL-only, or something to that effect. The poison-pill was designed when Trolltech first sold away Qt, if i remember correctly.
