As has been said no system is 100% unbreakable but some vendors such as Dallas semiconductors do make secure 8052 compatable devices which incorporate some fairly basic encryption at the prom.
But as we say to all our customers its really a matter of how valuable you think your data is and how much effort anyone is going to be willing to put into reading it.
If they are really keen they will skim the top off the memory and read it with an electron microscope.