As one of the current Sequel Pro developers I can backup jabkobob on this one, it's not an easy task. It's not that we're ignoring the requests we get to add support for PostgreSQL or any other database system for that matter, it's simply a matter of time, of which most of the core devs don't have just now. We have a lot of plans around this area (as well as others) for the app, but it's going to take time.
Thanks for everyone's support.
(Disclaimer: I haven't been involved much with SP for almost a year now, so possibly there has been progress in that time)