precision of epoch

I'd like to convert timestamps without timezone to unix epoch values
with at least microseconds resolution.
but when i do e.g.:
select extract (epoch from timestamp without time zone 'Thu 14 Jun
05:58:09.929994 2007');

i get:
1181793489.92999

so i loose the last digit. I'd expect 1181793489.929994

That's as well the behaviour, when I use columns of a real table. I
seached the docs for a precsion value and even tried timestamp(6) as
well but with no success. What am I missing here ?

Gerhard