Thanks for the comment. I’d have to disagree with you that CSS is the only answer. I see three good reasons for using the <p> element:

Using the <p> element allows the spacing to be maintained when your post is being viewed in feedreaders.
If you’re using WordPress MU, CSS classes aren’t allowed — and stylesheet editing isn’t either.
If your lists contain enough content to warrant padding for readability purposes, then it’s probably a paragraph, which means that the <p> element is entirely semantically-correct.

The best way to do so is, definitely, to add a CSS rule so that li elements are padded. Either all your lists within posts, or just the <ul class=”spaced”> ones.