This is an interesting concept...
AFAIK, it's normal to take your shoes off when you go to someone's house - at least, it is where I live. I'm sure some people where I live do leave their shoes on but even when a hostess has people over and says "Don't worry about it, leave your shoes on" people still say "Don't be silly" and they kick them off. Of course our weather is not the greatest in most seasons so maybe that has something to do with it. Guess I'd have to have a "Leave your shoes on" party!
Could you make an invitation in the shape of something foot-related rather than an actual sock? Do the invite in the shape of a foot file or other pedicure implement or like a mock Dr. Scholl's (sp?) foot powder bottle and make the writing on the bottle the wording of the invitation and use the yellow and blue colours of the foot powder bottle...think outside the shoe box!
Maybe my ideas are lame but they might help someone else come up with a better idea.
Kelly Bee