What can you tell me about the Hebrew word for day (yom) used in the Genesis 1 creation account? What is the proof for it being a literal day?

You asked about the use of the Hebrew word for day—yom. First, let's go back and list a few hermeneutical principles, then apply them: God communicates through language. Human language consists of words used in a specific context that relate to the entire reality around us. When Jesus and the New Testament writers referenced Old Testament passages, they employed what is often referred to as the Literal Principle (we also routinely employ this method in our everyday conversations): Seek the ordinary meaning of the language. Identify type of language: Poetry or prose; figurative or literal. Begin with the assumption that the writer is saying something to be taken in its literal meaning. Seek a single meaning of the text. The structure of a narrative is the single greatest tool the writer has to convey his central message. We observe what a writer selects and how he arranges his material to best discover the central message.

When we apply these principles to Genesis 1, there aren't a lot of possible interpretations. In fact, I don't think that God could have more clearly indicated 7 ordinary days. Let me explain:

First of all, Genesis 1 is clearly historical narrative.
Yom (day) used with a numeric adjective (Day 1, second day, etc.) always means an ordinary day everywhere else in Scripture.
"Evening and morning" (used for each of the six days of creation) together always mean an ordinary day everywhere else in Scripture.
"Evening" used in association with yom always mean an ordinary day everywhere else in Scripture.
"Morning" used in association with yom always mean an ordinary day everywhere else in Scripture.
In Genesis 1:5, yom occurs in context with the word "night." Everywhere else in the Scripture it means an ordinary day.
Even the use of the word "light" in 1:5 determines the meaning as an ordinary day.
There are words in biblical Hebrew (such as olam or qedem) that are very suitable for communicating long periods of time, or indefinite time. Yet these words were not selected.

Other passages confirm the interpretation of yom in Genesis 1 to be ordinary days:

Exodus 20:11, 31:12-17 shows 6 ordinary days. The plural of yom can be used to communicate a longer period of time (e.g., "in those days"). Adding a number here would make no sense. Clearly, in Exodus 20:11 where a number is used with days, it unambiguously refers to 6 earth-rotation days.
Jesus said that Adam and Eve were from the beginning of creation (Mark 10:6). If the days of Genesis 1 are epochs, then Adam and Eve came at the end, not the beginning of creation.
Adam was created on day 6, lived through day 7, then died when he was 930 years old. If days 6 and 7 were epochs, this would make no sense.

All this to say, when we take a literal approach to Genesis 1, I don't think it is possible for yom to be anything but ordinary days without violating the clear meaning of the text.