Actually Existing has a relatively penetrative analysis of the memo advising Downing Street on extraordinary rendition leaked to the New Statesman. So far as I can see from having read the memo, it is, as Phil says, certain that at least one incident of rendition - that is, extra-judicial extradition - took place, and probable - "[t]he papers we have unearthed so far suggest there could be more such cases" - that there are more. Rendition, even in cases where torture is not involved, is usually illegal, simply because international treaties ban expelling people without due legal process. Cooperation with such actions would therefore usually be illegal. Although obviously rendition to a state which was likely to torture or otherwise ill-treat the person who had been rendered to it makes the practice much more obviously wrong, rendition of itself, even without torture, is still obviously wrong. It's kidnapping people.
Kidnapping people is, fairly unambiguously, wrong. Kidnapping people so as to send them where they will be tortured is certainly worse, but that doesn't make it acceptable to kidnap them. There are extradition procedures for a reason, just as there are bail hearings for a reason: to prevent people who have been convicted of no crime from suffering a loss of their liberty without good, publicly available, grounds. To bypass or subvert those procedures is to create a sphere of arbitrary power, where the conduct of the state does not have to be justified, is unaccountable. It is a violation of the rule of law. It is the introduction of a category of exception, of the excluded. Those whom the state claims present a threat to its citizens are no longer judged deserving of the protections which stand as a barrier against the possibility of the state itself becoming a threat to its citizens. The claims of order trump all. Neither, of course, may citizens themselves stand in judgement of those claims of order: they are reduced to ciphers, in whose name action is taken, but who at no time are to have the capacity to hold those who take the actions accountable. One might wonder what was it was that made it so important to secure order for these weak, stumbling automata, to make such efforts, such sacrifices.
Such a situation is, as Rochenko points out, rather dangerous. It panders to visions of unlimited power, a utopian sublimation of politics where those with the will to bear it will hold back and then turn the tides of darkness, prepared to do whatever it takes, stopping at nothing. Those, I fear, are always likely to be present in political discourse. What is more worrying is the normalisation of this as a discourse, as a political situation. Citizens are disempowered, deprived of information and oversight, unable to take the hard choices that are necessary for their security. Liberties are restricted, so as to give the powers to the state and its organs necessary to combat the existential threat, a threat which never seems to diminish, is endlessly fertile. Seem oddly familiar? I thought so.