Product Protection is an imporant consideration during product planning and often gets implemented as an after-thought in most environements.  More structured environments that use CMMI or Lean Product Development tend to include this factor in the initial requirements.  

When we talk about product protection for OEM MCU based products, really the key issues are trying to protect the software intellectual property (IP) and trying to protect the hardware IP.  Let's consider each one in turn.

Software IP is best protected in flash by having mechanisms which don't allow the flash to be read.  If the flash can be easily read, it is simple enough to duplicate the hardware, steal the contents of the flash, flash new parts and sell these cloned products.  To avoid this it is best to have parts that can be locked.  Another similar approach is to use an encrypted serial EPROM to provide the code, with the code only loaded by the MCU when the correct key is used.  This really another means of locking in hardware.

If your MCU flash cannot be locked, you can make it easier to detect who stole the product software using embedded serial numbers and periodic software checks, but you cannot prevent the theft of the software.  The serial number can then be used to eliminate warrantee repair costs.

Hardware IP is best protected by potting devices, using multi-layer boards, scrubbing parts and using multi-chip modules.  When a circuit assembly is potted, the entire circuit board is encased in plastic or acrylic of some kind.  The resultant circuit is much harder to recover from this potting.  Multi-layer boards are more difficult to duplicate making theft more difficult.  Eliminating part numbers from the parts or scrubbing the parts also makes it more difficult to steal the design.  

A more elegant solution is to use multi-chip modules or MCMs.  In this case the parts are inside a package and wire bonded together.  It could also include analog components and may even use FR4 as a carrier.  Eventually these components are completely encased, again to make it difficult to duplicate.  This is much more secure that surface mount PCBs that are potted.

Product protection is important for OEM designs to avoid product theft.