The area around Pailin is still mined as a result of the Khmer Rouge precense.
They held out ehre for years after the fall of the Khmer Rouge in 1979, with silent backing of the thai army, and in order to keep the cambodian army and police out they mined the area.
Today a lot of work is being done to demine the area, but it´s a huge job and mines are still out in the bushes.
Good thing is that they have now posted the area well with warning signs in the places that are not cleared yet, so you have no probalems as a visitor as long as you respect the signs.
There is a border crossing to Thailand at Pailin, but it is being used very little by western travellers because there is next to no public transport to there from the rest of Cambodia.
But it´s a nice alternative to the more busy border crossings, especially if you are travelling by bicycle like me and prefer less traffic.
The border crossing is pretty straight forward and the lines are very short at the border as there are generally few people crossing there.
The main attraction at the border seems to be some casinos that are popular with thais crossing in to Cambodia to gamble.
Be ware though that services are quite limited at the border.
There is very little in way of exchange facilities, hotels and public transport.
This is bound to get better soon as the world get´s to know about this recently opened border crossing, but when i was there in december 2011 i could not change money at the border and the nearest ATM was around 35 kilometers from the border.
I was by bicycle so i just cycled to the ATM, but make sure you are prepared for that if you prefer to chose this charming little border crossing.