The following bug has been logged online:

Bug reference: 1993
Logged by: Nicholas
Email address: hb (AT) pg (DOT) x256.org
PostgreSQL version: 8.0.3,8.0.4,8.1
Operating system: Gentoo Linux
Description: Adding/subtracting negative time intervals changes time
zone of result
Details:

spatula ~ # psql -U postgres
Welcome to psql 8.1beta1, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help with psql commands
\g or terminate with semicolon to execute query
\q to quit

postgres=# SELECT VERSION();
version
----------------------------------------------------------------------------
----------------------------------------------------------
PostgreSQL 8.1beta1 on i686-pc-linux-gnu, compiled by GCC
i686-pc-linux-gnu-gcc (GCC) 3.3.6 (Gentoo 3.3.6, ssp-3.3.6-1.0, pie-8.7.8)
(1 row)

postgres=# SELECT NOW()-interval '1 week';
?column?
-------------------------------
2005-10-17 08:52:37.355219+10
(1 row)

postgres=# SELECT NOW()-interval '-1 week';
?column?
-------------------------------
2005-10-31 08:52:39.021583+11
(1 row)

postgres=#

---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match