Ah yes - I forgot about the August high season... For
francophile, if you still want to travel from PP to Siem Reap, do note that going by bus or taxi is quite nice and doesn't take that long -- only around 5-6 hours (including rest stops). If you were willing to get up early enough to take a 6:45 am flight (which means getting up around 4:30 or 5 am), you could arrange a taxi and be in Siem Reap comfortably before noon.
As a plus, you get to pass through the town of Skuon, famous for a local delicacy: fried tarantulas! (See photos and text
here and a more personalized description
here ; also available in plusher surroundings at the quite good Phnom Penh restaurant Romdeng -- see website
here .) And you get to see some of the "real" Cambodia -- that is, the towns, villages and farms that are outside of the economic boom areas of Phnom Penh and Siem Reap. And the rice paddies are beautiful at this time of year.
However you end up traveling, I hope you enjoy your time here.
Michael