Chuckleluck 0 Light Poster

Hello,
I would like your opinion on my collision detection & jumping code. Is it efficient enough to be used in a full-blown game? Any ideas on how I could improve the efficiency of the code? Any bugs?

Please reply with (constructive) criticism, comments & suggestions.
Link: http://dl.dropbox.com/u/56274235/Working%20Collision%20Detection%20%26%20Jump.zip
Thank you for your time.