I really don’t like the underscore [ _ ] character!
- Please don’t use it in your hostnames, although it appears to work, it’s actually not valid and generates warnings and errors.
- Please don’t use it in your filenames, filesystems can handle files/dirs with whitespace these days.
- Please don’t ask ‘underscore or dash?’ when I say ‘dash’.
If I wanted the underscore, I would say so.
Don't fear the penguin 
So you don’t want to mail me at dimitrios_stergiou@my_blog.nihil_novo.eu?
lucky me, I know your other email, the one without underscores :p
I’m ready to drop the underscore character, when every client downloading files with a space character in their names stops converting them to %20s. For the sake of the average non-competent user…
I think it goes the other way around.. When every browser converts them back to whitespace, instead of keeping %20s in place (because web servers do that in URLs).
That’s my point; because one can’t trust that space characters won’t be converted to %20s on web and ftp servers, (s)he prefers to use underscores in filenames instead. Plain «concatenation» without word delimeters, is not always convenient -not even on servers with case sensitivity, where one could the trick of initial letter capitalization of each «word» in a filename.
I said clients, not just browsers, on purpose. It’s thousands of them out there. Add anchor links in any xml/html document, and you get a huuuge problem.
Here is a solution to process files that contain whitespace in their name (change the $IFS) http://cant-remember-everything.blogspot.com/2010/06/batch-rename-files-using-their.html