unixtime -> timestamp with time zone

I have a lot of date data in unixtime format, which I'd like to store in
postgres as a timestamp with time zone. In 7.2 I had been able to say:

datetime(1038812385) at time zone 'pst'

which worked great, but in 7.3 this is telling me:

ERROR: Function datetime(integer) does not exist

How can I convert from unixtime to timestamp with time zone? Preferably
without specifying the exact time zone and just using the system default,
instead of the default of utc.