250.000 Twitter accounts were compromised yesterday. It's nothing compared to LinkedIn 6.5 million in June 2012, but it shows that even big guns have security holes and that it's really good to have a strong password.
So not using only letter and numbers is a good idea.
Special characters from ASCII table are 100% safe to use (I still don't know why some developers are still sticking with iso-8859-1 encoding):
The Twitter blog gives security tips about passwords and it reminded me that no one ever mentions one character ...
I haven't heard of or came across one example where password was trimmed, fiddled with or validated to the extent of preventing you from using certain characters, so I think it's safe to use a space in passwords and it provides even more security.
I don't know if brute force or any other method for decrypting password includes space as a possible character, but I doubt it.
P.S.: I wonder if it would be possible to use 'tab' or any other ASCII character from 0 to 1F