How to get from Phnom Phen to Phu Quoc in one day?

You can bus it to Sihanoukville or the Ha Tien border crossing at xa xia and then pop across the border. And a 1 and 1/2 hour ferry ride to the island. you could have a problem here. I think that the only ferry to Phu Quoc leaves at 8:30 in the morning. Make sure you have your viet visa in order first.
Probably the fastest. A plane to Saigon and another to Phu Quoc.
Or you could bus to Saigon as well and then fly in.
Sianoukville is cool and the bus trip is not to difficult. You can probably get a bus directly to the Ha Tien border crossings and just pop across. Other wise its a shuttle from SV. The ferrys vary in time and price. check out this site for some answers.
http://www.phuquocmap.com/phuquocsea.htm
This site aslo has data on flight to Phu Quoc and it seems that Viet air has a lock on that. so it goes.
ferry to Phu Quoc tele number is 3959060 or 077-3996456