I actually don't see additional tags in author-created lists. At least in 2011 SP1, authors using the bullet option in a rich text format area creates unordered lists.
<p>As an author, I want to create a list:</p>
<li>This is the first item in my list</li>
<li>And this is my second list item</li>
Earlier versions of SDL Tridion 2011 had different markup behavior based on browser, specifically on paragraphs. The fix was indeed through an update to the default XSLT filter. Contact Support if you're experiencing unexpected behavior, especially across different browsers.
How to Manually Adjust
When authors have unwanted
<p> tags within lists, they can change
Section Type to
Body Text. They don't need to know markup; they just be able to know when selecting a List Item, they may or may not want Paragraphs inside them.
Neil's approach with custom styles is a nice way to add different "types" of lists, especially if slicing refers to the desired HTML markup. Do consider "templating" the markup based on component presentation (changing components or fields based on a given schema and/or template).
How to Choose an Approach
Be careful with changing the stored CMS-side content. I prefer RTF Filters when the markup is "wrong", but templating options when you know it will change in the next site design change.
Consider a Rich Text Format Area XSLT Filter to restrict or clean up classes, styles, and markup (even when authors have access to a field's source). I would consider these semantic changes and fixes. Follow up on Puntero's answer, especially if the "something" CSS class is a non-changing, functional feature of the lists in a given field.
Consider template code to add site-specific classes, styles, and markup to match design choices that may likely change.
As much as possible, avoid baking in style-specific requirements into rich text fields. Though we might use styles (css classes), it's still a semantic selection to authors, meaning it has some business meaning.