I really wondered: Some guys have made mods for all kind of games, included Portal 2...
Now, how come Portal 2 doesn't have surf maps?
I mean, the theme is perfect; surfing through Aperture Science and use momentum rules and stuff to solve it!
How cool wouldn't that have been,and on Co-op friends could have a surf race to see who comes first and, then solve the rest of the chamber..
Someone should make this...
From a quick google, sv_airaccelerate doesn't exist in portal 2 and that seems to be the main reason.
However it does somewhat possible as proved here.
EDIT: further googling reveals that it is possible to edit sv_airaccelerate using sourcemod, so it is possible, but still is probably the main factor as to why surf maps don't exist.