While it is possible to apply rich text formatting to whole fields by modifying the CSL citation style, Zotero 2.0 doesn't support rich text formatting within fields.

However, one can use the workaround described in this forum post, which relies on a set of HTML-like tags for italics, bold, _subscript, ^superscript and smallcaps. While these tags have to be applied manually, this approach can be combined with a macro for your text editor to automatically remove the tags and apply the formatting (a macro for Microsoft Word is available here).

Zotero 2.1 (currently in beta) will automatically identify these tags and apply the desired formatting in the bibliographic output.