Here's another wild guess:

http://www.postgresql.org/docs/8.1/interactive/release.html#RELEASE-8-1-9


    E.1.2. Changes


    ....


    Fix POSIX-style timezone specs to follow new USA DST rules (Tom)


TIme zone rules changed in 8.1.9, and if the timestamps in question are 
between 3/11/07 and 4/1/07, then you got bit by this bug.  You can 
probably backpatch the 3.1.8 installation to have the correct timezone 
-- or just upgrade to 3.1.9, it's probably easier than patching.


Bill Moran wrote:
> Postgresl 8.1.9 replicating to 8.1.8 using Slony 1.2.6.
>
> I have a number of "timestamp with time zone" columns that simply will
> not replicate correctly and I'm stumped to the point of insanity as
> to why.
>
> On both systems, the database is set to est5edt timezone, yet some
> rows (not all, mind you) change the timezone from edt to est when
> the data is replicated.
>
> It's always the same rows, but not all rows in the table.
>
> This has obviously got something to do with the data, but I'm befuddled
> as to what to do about it, or even how to determine the exact nature
> of the problem.  New rows are being added to this table all the time
> and they are replicated correctly, it seems as if it's always the same
> rows that magically switch timezones during replication.
>
> I've gone so far as to completely deinstall Slony from this database and
> rebuild the Slony config from scratch, and it still does the exact same
> thing.
>
> Any help will be met with great appreciation.
>
>