Remove Sticky Residue from Non Slip Items in Bathtub

Remove sticky residue from tape or other self-stick items like bumper stickers (and non-slip appliqués in bathtubs) with WD-40.

Steps

Use WD-40. It contains petroleum distillates that will dissolve the adhesive left behind by tape, bumper stickers, and other self-adhesive appliqués. It says right on the can that it will remove grease, tar, and adhesives.
Test spray a small area in a spot that won't be easily noticed and see if the WD-40 will harm the surface. You will probably be ok using it on porcelain cast-iron tubs, but may not be ok for plastic or fiberglass tubs. Always test first.
Once you have tested the surface and everything seems ok, spray a small area where the adhesive needs to be removed and let it soak a while. You can use a plastic scraper (or an old credit card) to help scrape the adhesive off without scratching the surface. Wipe up with paper towels or similar absorbent cloth.
Do small sections at a time. WD-40 has warnings on the can about proper ventilation, flammability, and the danger of breathing the vapors. Read the warnings and follow their advice.
Remember that WD-40's primary purpose is to be a lubricant. Do not attempt to use the bathtub until the WD-40 has been washed away. (Try using Soft Scrub). The bathtub will be very slippery until the WD-40 gets washed away and you could easily slip and fall, injuring yourself if you don't clean the WD-40 thoroughly from the tub surface.

Be prepared to throw out rubber gloves after using them
Clean the surface thoroughly after using WD-40 to remove the WD-40 residue.
Use plastic scraper to help remove adhesive
Do small sections at a time
Cold water works much better to rinse the WD-40... The hot water seems to adhere the WD-40 to the bathtub...