Kernel = 2.6.17-1.2187_FC5
Applies to passwords >~16 characters
Mount works if password is entered at password prompt. 
Mount fails if same password is passed on the command line as password=${PASSWORD} shell variable.
Password matches regexp /[A-Za-z0-9]/ so not unusual just long.

From my naive coding I think the line in mount.cifs.c ...
} else if (strnlen(value, 17) < 17) {

is where the error arises. I note higher up in the mount.cifs.c code the password is allowed to be up to 64 characters.