In my opinion, any patch should be committable to core once it works on mysql and has a decent hope (use common sense) to work on postgresql.
Kind of like saying: "Software really only needs to support Windows. The other platforms don't matter." Except in this case our "Windows" is only 30% of the market. You need to understand that MySQL is broken in a lot of ways. A lot of the "missing support for pgsql" bugs are actually "we are depending on extremely broken and non-portable mysql behavior" bugs. If we go down the route of accepting MySQL-only code, it will be very difficult to get out.
Prime example: http://drupal.org/node/146466 this is the most important patch we have currently as it makes Drupal search speedy and nice. And it is more or less on hold just because noone is sure whether the postgresql update works or not.
Actually, that patch was broken for MySQL too (see http://drupal.org/node/192348) and got committed anyway because no one tested its upgrade path. Separate rant. Thanks, Barry