I'm trying to think of a way to visually depict the meaning of "wish" or "wishlist". I'd prefer to stay more on the side of "wish" than "list" as I'd rather avoid an icon of a list.
If what you want is a button that people press so that they can come back to a product later, that's very similar to 'favouriting' something: storing it to come back to it later.
Stars are very often used here: from a usabilty point of view, a simple star icon will probably get across the idea "Press here to come back to this later" more efficiently than anything else.
To specify that this stars products, or that people can buy you it as a gift, you could combine ideas:
Context should be enough to get across whether this is "add to wishlist" or "go to wishlist": if it's next to a product, it's to do with that product, if it's next to your username, "my account", "log out", "basket" etc at the top, it's a page to do with that account.
For wishlists, I would personally suggest using either a star (already suggested), or a heart.
Since a product you add to your wishlist is something you wish for, and the wish comes from your heart, it can be argued that a heart icon can be a good choice for a wishlist or favourite list.
Add to wishlist - heart with a "plus" icon overlaid, grey background to show not wishlisted yet.
Wishlisted - red heart without the "plus" icon.
Hope this helps.
I saw Aladdin's lamp for that once (I think on Amazon).
A larger version, from iconshock (commercial, therefore the nag lines):
Personally, I would add stars and sparkles like in the first one.
A gift box icon like this would work:
you could represent it with a cupcake and a candle (or just the candle)?
When it's not selected it can just be a silhouette of an unlit cupcake (or just the candle), but when it's elected the candle can be lit with a bit more color/form.