TARGET = myappname
TEMPLATE = app
SOURCES += qtmain.cpp \
    ... \ ...\ ...
HEADERS += ...
FORMS += ...

unix {
    isEmpty(PREFIX) {
        PREFIX = /usr/local
    }
    # - Before BINDIR = $$PREFIX/bin
     #After:
	BINDIR = /opt/maemo
    DATADIR = $$PREFIX/share

    DEFINES += DATADIR=\\\"$$DATADIR\\\" PKGDATADIR=\\\"$$PKGDATADIR\\\"

    # MAKE INSTALL
    ...
}

...
(Before)
Exec=/usr/bin/myapp
(After)
Exec=/opt/maemo/myapp

...