ListGrid - CellFormatter conditionally falling to default behavior: how to?

Hi all,
I’m wondering how to implement a CellFormatter that operates its formatting logic only under certain conditions, otherwise falling back to default formatting.
More specifically I’ve not yet been able to find a way to call the default formatting routines (and I don’t want to reimplement them, since they work great).
I guess they are somewhat derived from SimpleType’s machinery, but I don’t have the whole picture.

Cheers
Davide

Using SmartClient Version: SC_SNAPSHOT-2010-12-31/LGPL Development Only (built 2010-12-31)