I think this could probably be due to the cable problems in Asia as of. My hits and revenue have fallen since the 27th. The day after the earthquake.
On the average it seems that RPC drops by 10%+ per month. I would assume this will continue to happen until it normalizes with Adsense rates. Publishers may complain about the drop in RPC, but for a good percentage of sites, at the end of the day, YPN still pays more than Adsense. Those that actually abandon YPN, will get replaced easily by all those waiting ever so impatiently to get in. So, the trend will continue. All you can do as a long time YPN publisher is to consider yourself lucky to have been able to been part of the program and prepare for life after the YPN boon. Good Luck