Once a design is on the site will it ever be removed?

I would have to say "it's possible" since there have been a few that disappeared and never came back. So I recommend that you get them if you think that you might want them someday and save them in a safe place (like burned to a CD). That way, you are covered if one you like were to suddenly not be there anymore. =]

hazelgrace, there have been a few disappear for reasons unknown to me.