an update on this topic, as we just did that travel from Phnom Penh to Bangkok by bus yesterday, January 8th 2012.
Quick version: 16-19US$/person.
Takes the whole day, 6:30am to 10:30 pm door-to-door, in our case.
Reasonably hectic, be warned that after crossing to the Thai side, the "bus" is a minivan and you need to make extra-certain that the driver knows where to drop you, and reconfirm as you get close to Bangkok city.
read on for a detailed account:
Ticket: we bought it online via Sinh Tourist, as we had good experiences with them in Vietnam. Not as good in Thailand, read on... It was 18US$ / person, plus about 2$ of service charge, so 38US$ total for us.
During the journey, talking to other travelers we found that buying directly from Capitol Tours was only 16US$ per person.
The journey on Cambodian side:
The ticket says: "rendez-vous at 6:00 am at the Sinh Tourist office in Phnom Penh".
when we got there at 5:45 am, we did the check-in at the office, but the guy at the counter put us on a tuktuk (free of charge) which brought us to another location, where the regular "Capitol Tour" bus was waiting.
So the ticket we got was for the regular scheduled Capitol Tour bus, departing at 6:30am.
The tuktuk driver helped us to exchange our "Sinh Tourist" receipt for the "Capitol" ticket, which is needed to board the bus, and after 35 mins waiting we left Phnom Penh at 6:45am.
The bus is ok, a standard tourist bus seating about 55 ppl, no luxury and frills, no free bottle of water. The road was ok, not too bumpy. as always, seating at the back row is bound to be more bumpy than seating in the front rows.
The bus did several pauses for snack, lunch, and fuel refill; every 90 minutes on average. we got to the Poi Pet border around 2:30 pm.
There a Capitol Tours staff, who was in the bus with us, gave us tags to get into the next bus after crossing the border into Thailand.
We disembarked from the bus with our luggage.
Locals with wheel-carts were already loading our bags and asking us 100 THB (thai bhats; about 2,5 euros) to carry our bags to the border crossing, 250m further, but we sa about 8 years ago