I have an idea. Since people are probably going to write their messages a little longer than they were on Justin's card, how about making it a booklet rathar than a card? We could take several pieces of poster board and cut them in pieces (about the size of a standard photo album) and make each piece a separate page. It could be the MITM memory album compiled by members of the MITMVC.
On the front we would have a nice design, then on the inside we would include all of our sigs/messages. In addition, we could put some of our favorite memories from MITM. These would be things like a few favorite quote and pictures from each season (like 1 quote and pic per page, then put people's sigs/messages around them--think in terms of a scrapbook.) Everyone could submit some of their fav quotes/pic, then we could vote on which ones to actually include. We might also want to include artwork from members of the forum to decorate the pages--maybe forum sigs people have made themselves (or some other type of MITM related artword that members made) since we have so many creative people here.
By making it a booklet, we could use as many pages as we need to fit all this stuff in, then just bind them together in some way. That doesn't mean that people should write 3-4 page messages--I'm not paying to ship a novel! But something like what Mike wrote (No more than a paragraph--depending on the size of your writing.) What do you all think?