Didier,
Sorry for letting this languish. I have applied the portion of the patch that related to angular units. But the portion that related to linear units seems questionable. It certainly broke the autotest/gcore/gtiff_write.py test script:
Script: gcore/gtiff_write.py
TEST: SetProjection: byte.tif ... fail
Did not get expected projection reference.
Basically it reversed the conversion of feet to meters and I'm not convinced it should have. So I'd like you to file a new ticket specifically addressing linear units in GeoTIFF if you are convinced that it is still broken.
The reduced patch was applied in trunk (r15501) and 1.5 branch (r15502). I also added a test suite item for the angular units issue using your test file (r15503).