Eaten there 4 times in the past 18 months, excellent service and food every time. Business casual attire appeared to be the norm when we dined there, didn't see anyone in park attire.
I have only been once, but thought it was wonderful. I had the Chef's Tasting Menu, which was an expensive indulgence at $80, but totally worth it. It was such an adventure for the palette! The service was very good and on par for the prices. Full review here: http://www.disboards.com/showthread.php?p=41178101#post41178101
You do not need a jacket. I wouldn't go in shorts and a t-shirt, but I don't think they'd turn you away if you did. I'd recommend business casual attire.
I think the best way to get there from POR would be to take the bus to DHS and then walk or take the the boat to the Dolphin.