I updated to Mountain Lion on my Mac Mini Server on the evening of 12/12/2012 which runs PostgreSQL 9.2.1. I had the same problem when I found this question. When I did a search about this problem I found the following bug tracker on the matter.
http://sourceforge.net/tracker/?func=detail&aid=3570272&group_id=37132&atid=418980
One of the comments suggests to download the developer branch from github to get around this from https://github.com/phppgadmin/phppgadmin/zipball/master. I did this and copied the to /Library/Server/Web/Data/Sites/Default and renamed the folder. I modified $conf['servers'][0]['host'] to 127.0.0.1 in /conf/config.inc.php. I think I had to copy config.inc.php-dist. I have been successful in creating databases. As of 12/12 they had not released a stable version for 9.2. Hopefully they will soon.