date format

I could swear (but I don't think I can prove at this point) that 8.0
beta3 returned timestamps with milliseconds, like this:

YYYY-MM-DD HH24:MI:SS.MS

But 8.1 isn't doing that.  I see functions to format the date, but that
would require me to change all my existing SQL to specifically ask for
the milliseconds.  There's also the "set datestyle" thing, but the
granularity doesn't exist to specify an exact format such as the above.
Is there a way to tell postgres *exactly* what I want the *default* date
output format to be?