It's totally hit-and-miss, with many factors: degree of program "Hildonization", versions of API and libraries required, dependencies (sometimes just their names).

For instance, the old Maemo4 game "Gweled" installs just fine with "dpkg -install debname". But Advoid, a very recent one which should be naturally compatible as it's written in Python, does not - just because the Maemo4 .deb depends on "python25-runtime" whereas Maemo5 provides "python-runtime"...