Punctuation marks where invented to increase readability. So for God's sake, get rid of these semi-colons,colons; my eyes are bleeding.
If you really need the differentiation which shall be achieved with the semi-colons (comma and a non-comma-punctuation-mark), I would suggest parentheses:
Speakers at tomorrow's conference include James T. Smith (vice president of widgets at WidgetCo.), Mary Holmes (chief foo officer at FoobRUs), Sheldon DeVane (head of global placeholding at RiMelioraDies Inc.) and Lisa Catera (chief lending officer at Plugitin Motors).