I can create custom fields in the Setup -> Lead section, and I can seemingly lay them out into a custom configuration as well. However, after saving (etc.) viewing of a Lead page shows more or less default layout, and no new spatial layout whatsoever. See pics below.
That said, it does seem as if *some* changes do propagate through: for example I can make the "Lead Status" field mandatory, and when creating a lead Zoho *will* make sure I fell that field in (but there is *no* red asterisk adjacent to the field).
This is using Firefox 3.0.10. I have read a little in the forums suggesting IE is preferred for editing Zoho, but I have avoided that browser b/c for me, creation of a new lead therein leads to no visible data in the record whatsoever (see another recent post of mine; link not available yet).