This is exactly the problem: those who use Drupal on PostgreSQL must
be the ones who would resolve the SQL differences, test and debug patches,
...etc.

They are the ones that have a vested interest in it more than the rest
of us. All of us want a cross platform cross database CMS, but the testing
work must be done by those who use PostgreSQL.

Which brings us back to the original topic of this thread. And the fact there's a release critical issue[1] which has been waiting for 4 days and 16 hours now for a quick test on postgresql. I don't think I need to point out the irony of this situation.

1. http://drupal.org/node/164532 Improve table indices for common queries.