Inherits and triggers

I have a situation where I want all of my tables to have a timestamp field
that is updated to the
current_timestamp whenever a column in a row is updated. For a single table,
I could do this
with a trigger. So I thought of using table inheritance and created the
trigger on the base
table. But then, the trigger is not fired when updating rows in children
tables.

Does anyone knows of a better way to achieve this without having to create
the trigger
for every table? I'm writing a library and the tables are not known at the
time of writing
the library.

Any suggestions are appreciated!

Fangbing Wu