I gave up - for qt4 build I turned to scratchbox.

On "apt-get upgrade" it find hildon-initscripts version from SDK which is clearly for scratchbox (it tryes to remove mp-fremantle-002-pr) and "apt-get build-dep libqt4-dev" repeats the same unreasonable error message (see above).

Later I will compare the installed packages on both - scratchbox and N900 with qt4 debian/control but for now I think I am spending too much time just to fix a simple bug in small qt-based program.