timezone help?

I'm a bit new to PG, and having troubles with timestamps. The docs list:

timestamp [ (p) ] [ without time zone ] 8 bytes both date and time 4713 BC 5874897 AD 1 microsecond / 14 digits
timestamp [ (p) ] with time zone 8 bytes both date and time, with time zone 4713 BC 5874897 AD 1 microsecond / 14 digits

But an example of how to call to_timestamp either with or without a TZ would help.

My immediate problem is below..

create or replace function get_thermom_type(siteid integer, observationtime timestamp)

select get_thermom_type(1,to_timestamp('01-JAN-2011','DD-MON-YYYY') );
ERROR: function get_thermom_type(integer, timestamp with time zone) does not exist
LINE 1: select get_thermom_type(1,to_timestamp('01-JAN-2011','DD-MON...
^
HINT: No function matches the given name and argument types. You might need to add explicit type casts.

How do I tell to_timestamp to forget the about time zones? this didn't work either:

get_thermom_type(1,to_timestamp('01-JAN-2011 HH24:MI','DD-MON-YYYY 13:01') without time zone);
nor a bunch of other attempts.

Thanks for any help,

-Dave