Someone may have better insight than myself, but I've never liked the differences in browser display for list items. I noticed that in IE that playing with your margins or padding on a list item can cause the bullet to disappear. I suggest never using the standard bullet myself . I typically kill all list items (I am okay with Ordered Lists) standard settings and then give bulleted list a background image (of the bullet) instead. It gives greater control over the display.
It's something else. I removed the list styles, and the bullet does not show. Using images for bullets is another problem. Opera displays a tiny bullet, Firefox a big one at the top, and IE displays it at the bottom.
I am not suggesting using images for bullets, I am suggesting having NO bullet (no list style) and instead have a background image on your LI that is your bullet positioned to the far left with enough padding from the left that your text does not overlap. You are mimicing the concept of a bullet but not depending on how different browsers work with traditional bullets. Essential recreating the idea of bullets and using UL and LI to do it.