definately outside. Bin liners probably have all sorts of chemicals on them from the manufacturing process. Also theres the chance of smaller fish getting tangled in it I guess.
Another suggestion is posterboard. Just use double-sided tape to put it on the back. I have it on all of my tanks, and at 50 cents a sheet, it's dirt cheap.
I used a roll of black fablon from Homebase. It cost me £4 or something for the roll and I have plenty left over for another tank should I need it. I thought that it would be easier to apply and once it's on, it's on. Bin bags would surely fall off if the tape gets wet?
Here in Rhode Island I payed $12 for a 4x2 foot background. Nothing fancy just black on one side and blue on the other. I would have rather made a DIY one. Where do you guys get your backgrounds at such cheap prices?