(First, I'm assuming you meant that you covered a raw clay piece with foil, not that you "wrapped tin foil in the oven.")
Aluminum foil is very slick and won't react with polymer clay so it should just peel off. However, if the area of contact wasn't flat (e.g., you tightly-wrapped a dimensional clay piece with it for some reason), it could be more difficult to get the leverage to peel off easily. I'd assume you could still get it off though, just more fiddly and time-consuming.
You could at least *try* reheating the clay-and-foil to see if the slight softening of the clay could allow the contact to be less strong. And/or you could scrub on areas of clinging foil with a toothbrush or stronger abrasive, or perhaps something like Bon Ami powder (it's gentler than Comet, etc.).
As for the future, you can feel free to put a sheet of foil under a baking polymer clay piece or use pieces of foil to support an item while baking (either crumpled foil, or a foil layer over another supporting material) and it shouldn't stick. However!...any place the foil or any other material that's very smooth like glass, other smooth metals, etc, is in direct contact with heating polymer clay, a *shiny spot* will be left in that place because polymer clay softens slightly in heat and will take on the texture of any material it's in direct contact with.
The main time aluminum foil is used when curing polymer clay is simply to "tent" the clay, as mentioned. That will help keep the temp right next to the clay from being exposed to sudden higher temps and just moderates the temp so the clay underneath won't darken as easily. In that case though, the foil will not be actually touching the clay.
There are other materials, and often better ones, that can be used to protect the clay while baking however, either with partial covering (like tenting) or complete covering. Check out this page at my site for much more about those:
http://glassattic.com/polymer/baking.htm
(look under the category called "Darkening, Scorching" and also perhaps "Support During Baking")
And if you want baking surface materials that will easily separate from polymer clay, ordinary paper (without printing) works very well because it's porous (not very smooth or sealed) and it's texture is pretty much the same as cured polymer clay so won't show where there has been contact during heating. Other materials can work too and not show like tissues, polyester stuffing, piles of baking soda or cornstarch, etc. More info on those and more on the same Baking page as above, but under the "Materials To Bake On" category)
If you still want to use a larger amt of foil in contact with a dimensional raw clay piece and think it might stick, just use a release on the clay and/or foil. Releases could vary from oils or Vaseline to powders like cornstarch and more.