> Does anyone know which cereals are dairy-free? The ingredients in many of
them seem to be
> dairy free, but they rarely say Pareve--or any Kosher marking. Is that
because of cross contamination?

No, it is because of the lack of Kashrut supervision (which costs money,
complicates the process and is only important to about 0.1% of the world
population).

If you get a Kosher Mayo brand, check the marking.  The absence of a 'D'
(diary), 'DE' (diary equipment) and similar designations means it's parve.

> Also, are all forms of Mayonnaise Dairy free?

Do not assume, check the labels.