All, One observation from my last trip to PP in late December '08..
Everytime I went to exchange money, each of the four exchange shops I went to simply REFUSED to accept any old US$50's or US$100's.. no matter how clean, crip they were.. no go. Of course, I had a bunch of $20's which were of both the old and "new" designs.. but the exchangers did not care about my $20's or $10's.. they only refused to accept my "old" style $100's/$50's.
I've had this happen to me in Manila as well.. I exchanged once at SM Mall of Asia and they refused to accept some old $100's despite the fact that they were clean and crisp.. Girl just said "Sir, I am sorry we cannot accept old money here. Sorry Sir."
Usually when I go to Cambodia I take a few larger bills which I use (or exchange) to pay for my hotel bill and a bunch of smaller stuff like $20/$10 for day-to-day purchases.. A wad of $1's helps too..

As a frequent traveller in Asia, I normally always inspect my bills when cashing a check or getting foreign exchange.. If I am given a "ratty" bill I will politely refuse it and request another.. When in the US, I ask the teller for "new version currency please" and since they all know I travel to asia, they know why I ask for it and it's never a problem.

Being a cheap charlie its alway's better to give the taxi drivers a $1 note, the problem with a 5000 riel note is its just over a dollar and the 10000 is about 2$ 75c. Taxi drivers will not give you change from a cambodian riel note.

I always bring crisp new $100's whenever I travel, learned that a long time ago.

It's funny, the Mobil station around the corner from my house in the states wouldn't accept an old $100 bill from me either.