Hi truthseeker.
God sent a Messenger to every nation, as He said;
And we did not send a Messenger except with the language of his people to make it clear to them.. [Qur'an 14:5]
So according to Islamic belief - the aboriginals and the native americans did recieve a Messenger, who was from their own people, speaking their language. He told them to worship and serve God alone, and to be dutiful to Him. Then they either believed or disbelieved. It may be that they may have disbelieved, or believed.
If they disbelieved and persisted in evil and oppression = disobeying the Messenger, they would be destroyed.
If they believed after disbelief - God forgave them and gave them more goodness in this life, and a goodly reward in the one to come.
However, it is possible that if they did believe - generations after them strayed from the pure teachings of their Messenger, distorted his teachings, and began to overpraise their pious/good people. To the extent that they built buildings over their graves to honor them, and gradually overtime transformed the location of their graves to idols. So idolatory began again, as with most nations. So they believed in One supreme God, but they worshipped others along with Him to 'intercede' with God, when God had never commanded this.
God may have sent them many nations many Messengers'.
However, after a long chain of Messengers - 124,000 Prophets', God sent His final Messenger, Muhammad (peace be upon him.) He would be the final Messenger from a chain of other Messengers', calling to serving the One Supreme God without any intercessors. Direct worship to God, doing all that is good and forbidding all that is evil. With the final supreme guidance which would be suitable uptill the Final Hour on this Earth.
Since we humans are God's slaves and representatives on Earth, it would be our duty to travel to these lands and convey the teachings of God to the people. And this is why you will see that Islam spread to the native Americans even before the White/Caucasian race reached America (i.e. before Christopher Columbus.)
See;
http://islam.about.com/od/history/a/afammuslims.htm
Also, Islamic Justice spread far and wide so quick after the passing away of Prophet Muhammad. So that from the origin point of Medinah in Saudi Arabia, Islam spread to Western Spain and Eastern China within only 90years.
This shows that God wanted His guidance to spread across the globe at a really quick pace, since that would ensure that Justice spreads across the globe. However, it would be the duty of God's representatives - us humans - to spread the message, and when we slack in conveying the message - that is when justice is at a low in the world.
Just to end the discussion; God has set a Day when He will gather all of mankind after their death, and Judge them all individually. Each person will be rewarded or punished fairly. Those who believed in Him and His Messengers' [including God's slave and final Messenger Muhammad], they will be rewarded by God for doing good to please Him. Those who rejected the Message and did evil will be punished for their own evil.
Yet those who did not hear of the message - like the Native Americans, Aborigines, or any other people - they will be Tested on Judgment Day by God, and then He will either reward them or punish them based on His Justice, since He is the Most Just.
If you have any questions, please do ask. :)
Peace.