Who is the "us" mentioned in Isaiah 6:8?
In traditional Christian theology, this has typically been interpreted as a pre-Incarnation reference to the Holy Trinity. Certainly, John Wesley's Explanatory Notes take this view. While that is certainly a legitimate interpretation, I'm not convinced that it's accurate.
Isaiah of Jerusalem's ministry took place in the seventh and eighth centuries B.C. The world was unquestionably pagan at that time. It was taken for granted, in fact, that there existed many, many gods.
Other nations found Israel odd largely because they were at least officially devoted to one god, YHWH. But early Hebrew religion did not claim that YHWH was the only god; the claim was made simply that YHWH was the sovereign god, and that the Hebrews were allowed to worship no other god.
The belief that while there are many gods only one should be worshiped is called "henotheism" by scholars, "monolatry" by theologians. Personally, I prefer the former term, as it seems less aggressive and more merciful to our ancestors in the faith.
I believe that the Hebrews were orginally henotheistic...perhaps the first true henotheists in the world. Abraham, Isaac, Jacob, Moses...they all seem to have been willing to accept the idea that other gods existed (or at least other divine beings), but were adamant in their devotion to YHWH as their unique covenantal god.
The first of the Ten Commandments seems fairly henotheistic to me, as the sole worship of YHWH is strongly endorsed without denying the existence of other divine beings. Some of the more repulsive stories of sin in the Old Testament refer to human sacrifice (especially of children) to terrible gods such as Chemosh and Molech...evidence that Israel at times practiced actual polytheism, and not even true henotheism.
In short, I believe that to be a faithful Jew for many centuries meant being a faithful henotheist. Isaiah of Jerusalem certainly qualifies. I believe that when when the prophet hears and records God's words in Isaiah 6:8, he hears them through the lens of henotheism. God revealed his presence, truth, and salvation gradually, preparing us for his Incarnation.
When we read the words, therefore, "...who will go for us...", "us" refers to the other members of the heavenly court...the "hosts of heaven".
This is one reason why one of my favorite titles for God is "YHWH Sabaoth"... "the LORD of hosts" (used powerfully by Martin Luther in verse two of the first great Protestant anthem "A Mighty Fortess Is Our God"), and why I often use Bible translations which retain this translation, such as the RSV, NRSV, and ESV (the NIV and TNIV translate the phrase as "Sovereign LORD" which, while true, doesn't capture the full essence of its meaning).
It's also a reason why perhaps my favorite creation story in the Old Testament is Psalm 89, which is really a masterpiece of henotheistic thought, recounting the tale of "YHWH Elohim Sabaoth", the Divine Warrior, who presides over his heavenly court and defeats the monster of chaos ("Rahab") to construct the world. Very Near Eastern, very mythological, extremely beautiful (though, like the more famous stories in Genesis 1 and 2, not meant to be taken literally).
Henotheism is a very strong part of our faith heritage, but Hebrew religion did not remain henotheistic. At some point, likely during the period of the Babylonian captivity, the Jews made the transition from henotheism to monotheism. Psalm 89, formulated in the Davidic era (but likely perfected at a later date), portrays creation from a henotheistic perspective; the first of the two creation stories in Genesis, largely an Exile era work, is a masterpiece of monotheism.
Even in the Book of Isaiah, we can see the transition. Whereas Isaiah of Jerusalem (or "First Isaiah") operates from a henotheistic weltanschauung, "Second Isaiah" is decidely monotheistic, as these verses demonstrate. By the time Jesus was born, the Jews were strongly monotheistic...the first true monotheistic faith in the world (despite the highly inaccurate claims of Zoroastrianism scholars such as Mary Boyce).
What does this mean for us, practically? Not much. Judaism is today a monotheistic faith, as is Islam. Christianity is a modified monotheistic faith (though Trinitarian is a better term). Henotheism survives today among the Mormons and Jehovah's Witnesses, as well as among Zoroastrians. While a part of our faith history, it is not a part of the Church's present or future.
Still, as a lover of history and as someone who eagerly desires to understand how our faith grew into its current form, henotheism remains a fascinating subject for study.