as far as I know ALL atmel processors can be popped even the AVR series, The 89cXX chips are popped quite easily
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I wount tell you exactly but I will tell you all you need is a 74ls123 (also a few caps/res) for the delay and this gets hooked to the line that goes to the prog pin on your programmer goes to the input of the 123 and the output goes to the chip.. if you issue an erase command with the proper delay only the security fuse is reset.
(obviously there are otherways to accomplish the same thing, you can write 1 shot code into a micro but the 74ls123 is the easiest)

there are a few other things that can be done to make it more reliable, but this should put you on the right track.. 1 word of caution each processor version, 89c51, 89c52, ect all have different delay values so keep track of which processor you are using..

If you are interested in security stay away from intel parts... my suggestion is to use phillips otp parts.. I haven't had any luck dumping them.