I think the argument is that If eveyone lied in the same situation. everyone would expect people to lie and thus would not be deceived by the lying. The general concept seems to be to imagine that everyone does it, what would be the result.
Im not sure I agree with this argument. especially when it comes to deception. Some one who expects deception can still be deceived, depending on the skill of the deceiver and whether they know that the person being deceived knows the deceiver is trying to deceive them (run your mind over that twisted thought 
)
You also have the problem posed by Benjamin Constant (http://en.wikipedia.org/wiki/Categorical_imperative#Inquiring_murderer) who said that if deception was never permissible. It would be morally wrong to lie to a murderer asking on the whereabouts of his target. You must tell them the true location of the person they wish to murder.
Considering the universality of deception among not only humans, but all living organism. i find it unlikely that telling the truth in all circumstances would be a moral imperative.
Close summary. Here is the more nuanced version.
The only sure way to avoid deception is to not listen. If people are expected to try to deceive in a circumstance then their targets can be expected to ignore them. The deception is impossible since deception requires the target's attention.
I also am unsure if I agree with this particular argument. However I do find the form of the argument to be an appealing attempt at deriving moral truth.
My response to Benjamin is:
Since moral permissibly of lying results in all lies being ignored, if it were permissible to lie to the murderer then the murderer would ignore your lie. Thus lying results in saying nothing of notice. So our options are: truth, nothing or ignored lie. Ignored lie can be eliminated because it results in saying nothing. So we can chose between saying nothing or telling the truth. There is no perfect duty against saying nothing. Therefore one can say nothing and avoid lying.
Can one draw likelihood across the Is -> Ought barrier?
The part I disagree with from this is that it assumes both people have complete and accurate knowledge about the situation they're in and the relative likelihood of someone to deceive them, when this is almost never the case. Usually one is deceived when one thinks that one is in a situation where there would be none or minimal reasons for someone to lie to you. For example, an undercover agent infiltrates an enemy base and disguises himself as someone else. While most people given this information would expect the agent to lie about their identity, the people in the enemy base have no reason to assume that the agent is an enemy trying to deceive them. Hence, deception is carried out.
In the case of the murderer, it's possible that they sincerely believe that you would help them find their target, either because they think that they have convinced you that the target should be killed, or that you wouldn't lie if the murderer threatened to kill or hurt you too, etc. In other words, deception is certainly possible through lying when one does not have omniscient information about the person lying.
You might want to read Kant yourself. It is a difficult read and difficult to accurately summarize.
The only knowledge assumed is that everyone adopted the maxim. The relative likelihood of someone to deceive them is not needed since even if it were infinitesimally unlikely there would be no way to tell the difference between the true claim of the honest person and the false claim of the dishonest person. Thus both are ignored rendering dishonesty useless and dishonestly for deceit irrational.
Lets examine Kant's example:
We start with a maxim "I borrow money without intent to pay it back to obtain money."
If it were morally permissible for you then it is morally permissible for everyone.
Imagine a world where everyone had this maxim.
In such a world no one would be willing to lend money because no one would pay it back.
Everyone having the maxim has resulted in it being impossible to do.
This is what is meant by Kant as a contradiction in conception.
Now lets look at your case with the undercover agent.
We start with a maxim "I will lie about my identity to impersonate someone else."
If it were morally permissible for anyone then it is morally permissible for everyone.
Imagine a world where everyone had this maxim.
In such a world no one would trust people's claims about who they were.
Instead they would rely on other forms of security.
The willingness to lie to impersonate lead to impersonating someone requiring more than lies.
Everyone having the maxim has resulted in it being impossible to do.
This is what is meant by Kant as a contradiction in conception.
