The argument of silence, in the story of Rahab, is weak.
There is no "silence" in the argument presented. Rahab is
directly commended for hiding the spies.
Because she is not condemned for lying, and because she is commended for hiding them, does not mean that 'lying' is permissible.
First of all, no one is attempting to argue that "lying" in general is "permissible". We're not talking about trying to justify lying as a lifestyle, or even as something we do when we think it's the best response, but rather are talking about very specific and extreme circumstances that Rahab found herself in.
Secondly, the word "hiding" in itself implies
deceit. Simply hiding the spies was an act of
deceit. Here is the actual account of the situation from Joshua 2;
Joshua 2:2-7 (NIV)
The king of Jericho was told, “Look, some of the Israelites have come here tonight to spy out the land.” So the king of Jericho sent this message to Rahab: “Bring out the men who came to you and entered your house, because they have come to spy out the whole land.”
But the woman had taken the two men and hidden them. She said, “Yes, the men came to me, but I did not know where they had come from. At dusk, when it was time to close the city gate, they left. I don’t know which way they went. Go after them quickly. You may catch up with them.” (But she had taken them up to the roof and hidden them under the stalks of flax she had laid out on the roof.) So the men set out in pursuit of the spies on the road that leads to the fords of the Jordan, and as soon as the pursuers had gone out, the gate was shut.
There's a few things worth noting here; first, Rahab was
directly questioned about the spies whereabouts, and blatantly lied about it. She had
hidden the spies in an attempt to deceive the king's messengers
before she ever opened her mouth.
Secondly, everyone wants to talk about how Rahab was still a "heathen" and didn't know any better, but
what about the spies that Joshua sent who were
hiding? These spies were most certainly not "heathens" and they were most certainly complicit in Rahab's lie no matter how you look at it. Further, there is no record that those spies scolded her for lying. Why would they agree to be hidden, and place Rahab in a situation where she would have to choose between surrendering them or lying? Why wouldn't they just surrender themselves in faith and allow God to do a miracle for them as some have speculated Rahab should have done if she'd "known better"? The fact remains that those men of God were complicit in Rahab's lie.
If this were the end and Rahab were never mentioned again, we'd be left in to speculate on our own about her lie. But it isn't, and scripture is not "silent" on her actions, as we'll see.
The silence is equal to the silence on her harlotry. That is mentioned, at the same time as the lying, but not condemned..
If the account of Joshua 2 were the only account we had, that would be true.
However in the following 2 scriptures from the New Testament show just how "weak" the argument is that scripture is "silent" on her actions;
Hebrews 11:31 (NIV)
By faith the prostitute Rahab, because she welcomed the spies, was not killed with those who were disobedient.
James 2:25 (NIV)
In the same way, was not even Rahab the prostitute considered righteous for what she did when she gave lodging to the spies and sent them off in a different direction?
A few things to note here as well; first, you'll note that Rahab's harlotry is
not what is commended here, so to continue to bring it up is simply an attempt to distract from what is actually stated. James 2:25
specifically says that Rahab was considered righteous "
for what she did". What did she do? She gave lodging to the spies, hid them, and sent them off in a different direction.
How did she do that? By lying to the king's men and deceiving them.
Hebrews 11:31 goes so far as to say that Rahab acted
by faith because she "welcomed the spies".
It's a pretty "weak" argument to say that these scriptures do not
directly commend Rahab for her actions. We see
nowhere that her lie was something she should not have done, nor that she should be ashamed of it. To the contrary, scripture says she acted
by faith and was
considered righteous. It does not say, "she was a heathen had a long way to go". It says that she was considered righteous because she welcomed the spies, hid them (which clearly involved lying to the king's men) and sent them another way so that they wouldn't get caught.
How is following the scripture not following God if scripture is the Word of God? How can we say we are following Christ if we are disobeying His Word? Sure, Jesus was the authoritative interpreter of the law, which reflects the mind of God (and that is why He never lied, even to spare feelings), but we are not that free to flagrantly violate plain scripture just because we feel justified in doing so. It is not our place to determine when we should lie and when we should tell the truth. We are to simply tell the unembellished truth to others just as God in Christ has told the unembellished truth to us ... always. If you or anyone else lies to me to spare my feelings, how can I ever trust that you are being truthful with me about other things and not just justifying another lie? 
