Cleaning Soap Out of a Bathtub

Fabric liner:
Purchase a large, nylon net clothes bag [the sort that is used for collecting dirty clothes]. Place Liner and shower curtain inside, tie shut, then wash in the washing machine...the bag will protect the liner & curtain from abuse and you'll have sparkling clean items...available from your K-mart and Wal-marts ......
http://www.laundrybagstoreonline.com/form2834-yel.htm

Never use Comet or AJAX to clean unless you want to scour the surface away, this junk is nothing more than powdered 'sandpaper'.

Now get in the habit of wiping down the tub/shower enclosure each day... Purchase the following:
1) Squeege
2) Microfibre cloths
Squeege the surfaces first, then finish with a microfibre cloth wipe down, this will take about 3 minutes and your bath enclosure will be clean every day.

I'm assuming the bottom of the shower curtain actually has orange mildew on it. If it really is rust, ignore this suggestion. If your shower curtain is vinyl, wash it in warm water with a warm rinse (so it doesn't get cold and tear) with a couple of towels and a healthy dose of liquid bleach along with the normal detergent. Then just hang it back up on its hooks to dry. If you have a fabric liner as I do, you can wash it in any temperature the same way and it will kill the mildew. I have to do mine about once a month to keep it looking decent, but it's easy to do and a lot cheaper than buying a new liner.

P.S. Oops, I see you stated you have a fabric liner. If anyone else has a vinyl one, be sure to use warm water.

Try an old credit card to scrape the gunk off. If it's really built up: if you're REALLY VERY CAREFUL you can use a single blade razor held as parallel to the surface as possible to scrape the accumulation off. Needless to say, I wouldn't do this to a new tub, ours is 40 years old.

Try Comet or AJAX or get that shower cleaner that is on the commercial that sprays every 10 minutes or so. I can't recall the name of the product though.

Try baking soda on a sponge to clean the tub. Also try baking soda on the curtain, wet curtain area that needs stains removed and sprinkle on baking sode. Rub baking soda in, scrubbing back and forth. Launder as usual.