Nope. All depends on money which somebody is ready to pay for. You may set protected bits but it will cost about 2000Euro for reading protected chip anyway (for example, by phisical reading layer-by-layer). You may use Dallas high secure RAM-based uc, but there will be another man who unlock and decode it. Moreover, sometimes it is not need to read actual code; usually it is enough to see what does this code do -- using a logical analyzer helps with it.

In short: always remember - big brother looks for you ((=

Good days!