I used this and was able to upgrade from 1.2.15 to 1.2.18, but I did get 
this warning:

<stdin>:14: Possible unsupported PostgreSQL version 8.3, defaulting to 
8.1 support

I'm assuming for that I can ignore this.

Vick Khera wrote:
> On Thu, Dec 3, 2009 at 5:15 AM, Glyn Astill <glynastill at yahoo.co.uk> wrote:
>   
>> I took a quick peek in slony1_funcs.sql from the 1.2.18 release and the drop_if_exists functionality appears to be in there (although not in the slony1_funcs.vNN.sql scripts). I'm going to do a few more tests before upgrading our servers...
>>
>>     
>
> Once I added the lines
>
> drop function if exists @NAMESPACE at .ddlScript_complete (int4, text, int4);
> drop function if exists @NAMESPACE at .updateReloid (int4, int4);
>
> to slony1_funcs.sql it as was installed, my update succeeded.
>