Applying a function to selected element type in a mixed-type cell array

[Philip Nienhuis]

Suppose a cell array like:

arr = {10, int32(15), 'Txt'; '', 25, true; NaN, 'String', single(100)}

How can all numeric values efficiently (w/o for loops etc.) be converted 
to double, leaving the other cell array contents untouched?

Getting at something like  "arr(find (cellfun ("isnumeric", [arr])))" is 
easy enough, but the follow-up is a hazy to me.

Thanks,

Philip