Alrighty... So I've been researching the banner since I lasted posted and am afraid I don't have great news for you.
While I still think having a huge Muppets banner would be awesome, apparently the things aren't worth all that much. I wasn't able to find any past sales of a Muppet Treasure Island banner itself, but judging from the past sales and listing from similar type of movies and similar sized banners, it's value would be somewhere in the $100 range and you might sit on it for a while at that. $50 would probably move it quick, $150 would need a serious collector, so right around $100 sounds right to me. Of course, this is assuming it's in very, very close to mint condtion (ie zero rips, tears, creases, stains, etc...). Any damage or flaw at all is going to dramatically effect the price. So much so that IF there were any sort of damage you'd be talking $50 at the high end. I've always thought that was being a bit too picky personally, but that's the reality of most collectibles.
It seems like the fact that it's so big is what really hurts it's saleability. Most folks just don't have the room to display something that's six foot by four foot large and people who buy things like this definitely want to display them.
As far as actually selling it, my advice would be that unless you want to sell it to a serious Muppet fan (ie someone on a forum such as this), you're probably best to hold onto it for a while. Around about the time the new movie hits theaters there's going to be much more interest in all things Muppets, so it's much more likely you'd find a buyer at such a time. That said, a specialized forum for either Muppet fans or film memorabilia collectors would probably hold some interest year round. but, as I said before, it's going to take someone who's a serious collector/fan or someone with enough space to display something so large, so it might be a bit tougher sell than other Muppet or film collectibles.
Bottom line, it's definitely a cool piece, just not a particularly valueable piece. Something the right person (myself included if I had the money or the space) would love to have, but certainly not something for everyone.