I won't link them here, but there are services available who will un-encode files that were encoded by Zend Optimizer and IonCube Encoder. generally they charge between $1 and $5 dollars per script.
Zend and IonCube are companies who put a lot more R&D hours into an encoding product than 1 person could do by themself. If their encoding/obsfucation can be reverse engineered, then it's safe to say that yours probably can too. For anyone to say otherwise would be quite ignorant.
I understand your comments about a deterrant, but if it is that important for someone not to see your code, you should have your own dedicated server overseen by a trusted and skilled aministrator.
If your code is distributed, then you should definately encode it somehow, .. but to think it's going to be difficult for someone skilled to reverse engineer is nieve.