Probably the highest you can get it is with this Ritchey adjustable stem:
http://www.ritcheylogic.com/dyn_prodfamily.php?k=98295
30deg road stem, goes up to 120mm length:
http://www.ritcheylogic.com/dyn_prodfamily.php?k=98291
Not a huge Ritchey fan but I have the 30deg stem on my bike, easiest one to find. The adjustable goes up to 45deg. The others are 30deg.
Tallest I have found other than old Salsa steel stems.