I have some ideas that might correct the problem.

1. Use a separate programmer to program the AT89C2051 and then in the main circuit (that you happen to sell), tie the RESET line to ground, and leave it there. The drawback may be that you must turn off the device and then turn it on a few seconds later, but it prevents all programming actions.

According to the datasheet, if the RESET, P3.2, P3.5 and P3.7 are high, and P3.3 and P3.4 are low, then when XTAL1 is pulsed, bytes are read starting at address 0 from the micro. This might be how your potential code thief may be using it.

2. Learn the chip. If floats turn to highs (which might happen), then rip out the pins on the chip that are required to be low for code reading (see above paragraph). If floats turn to lows (which might not happen), then rip out the pins on the chip that are required to be high for code reading.

Just make sure that the ones you rip out are the ones you dont need.

By ripping out, I don't mean use pliers to pull them out of the IC, I mean bend the metal legs (or teeth if you want to call them that) of the IC and keep bending them until they snap off. It looks like you will get a clean cut.

Now it will be difficult for someone to get your code, unless they have a high amount of experience with soldering, AND they manage to modify your circuit by adding components, but if you make your circut without jumpers, or connectors, then the chances of your code being stolen go down.

And here is another interesting suggestion. Literally isolate your circuit.

By that, I mean do whatever it takes to prevent any external wires from making electrical contact with your chips. My idea (for a low cost) is to apply hot glue all over the entire board. The only thing you need to be concerned about here is that the temperature of the glue must not exceed the maximum tempurature any one chip can handle, or it might die.

Then if your code thief wants the code, (s)he will have to rip the glue out first. and Let me tell you, it will take at least a good 5 minutes.

Sometimes what I do (yes, I did this with my DC cord on my laptop) is connect wires together (in parallel), and instead of leaving the two polarized ends of the wires exposed, I apply hot glue all over them and wait until the glue turns solid.

For my glue, I melt a glue stick (for arts and crafts purposes) with the soldering iron LOL.