Just a side note: American Sign Langauge is different from British Sign Language and Manually Sign English (which is putting a sign to a word and keeping everything else the same; this doesn't constitute another langauge linguistically). ASL was actually derived from French Sign Language because the English wouldn't help the American deaf without a price.

And the first poster's statement is wrong. Different signed languages aren't as different as spoken languages. If a deaf person was immersed into a foreign deaf community, he/she would acclimate at a much quicker pace than a speaker. This is because sign language is a conceptual langauge, and most deaf-signers convey main ideas through mostly imagry rather than concrete grammar rules.