You are right. I forgot that. If one limit the move area of the lense one can use an FFP as well. In HL2 Lost Coast I noticed, when I walked up the rock that they changed the envmap at a certain place that it fits better to the changing environement.

http://www.puppenheim.org/
http://www.wuerfels-welt.de
Old Game Tutorial: http://www.puppenheim.org/MITutorial.zip
Free Game Editor: http://puppenheim.org/AlienalsWeihnachtsmann.zip
http://www.puppenheim.org/blender_starting_main_settings.txt