did you do it the wrong way?
I mean, the command goes like this:
ln -s source_directory symlink_name
where source_directory is the dir you want to be linked (/usr/src/linux-2.6.0) and symlink_name is the name of the symlink you want to create (/usr/src/linux). the symlink_name component must not already be there, because if it does, it would be overwritten and that's not done. so if you did
ln -s /usr/src/linux /usr/src/linux-2.6.0
then you got the error because a dir called /usr/src/linux-2.6.0 already exists (but on the other hand, there is no - or at least should not be - a thing called /usr/src/linux). so, to get this correct:
1) if you already have a symlink called /usr/src/linux, remove it:
(now this should print a line like this -> XXX: symbolink link to 'YYY' and if it does, then it is a symlink and can safely be erased. if you get an error which tells you that there is no such thing as /usr/src/linux, just move on, over the next step)
2) create the symlink to the 2.6.0 directory:
ln -s /usr/src/linux-2.6.0 /usr/src/linux
and you should be fine.