I just read quickly through the first two cables I found that originated from the Embaasy in Port-au-Prince. They are basically frank assessments made by US officials of Preval. As seems to be the case generally so far with the Wikileaks cables (only about 500 out of 268,000 released), the revelations are - as of yet - not that earth shattering (though we should not dismiss the importance of having widely held suspicions irrefutably confirmed) but I can see why these would seriously embarrass the US
I was struck more by the tone than the content of the Haiti cables - congratulatory on how well behaved and well managed Preval has been (at one point describing him approvingly as "neoliberal") - but still concerned that he can't be completely trusted (not listening enough to more pro-US advisors) and that he drives too hard a bargain - for example by appealing to Venezuela and Cuba and by being "nationalist".
Preval is described as being very worried that "politics" will pursue him after he leaves office. Reading between the lines a bit, Preval seems to fear that Haiti's drug trafficking issues could end up hounding him - i.e. get pinned on him. Preval is described as being "frustrated" with the US on this score. "Special Intelligence" has snooped into Preval's medications. Imagine the outrage if any country did that to the US president. Preval obviously knows that if he doesn't behave the US can pin corruption and drug trafficking on him - even if it he was uninvolved and powerless to stop it - and make his life hell after he leaves office.