This is a widely debated topic. The reality is the rate is somewhat irrelevant. Tax rates have no connection to tax revenue. There is a maximum amount of ones economic activity that can be seized by Gov't before economic activity changes.
In 1952 the highest tax rate was 92% and the Federal Gov't collected 18.36% of GDP.
In 1988 the highest rate was 28% and the gov't collected 18.79% of GDP.
Since the creation of the income tax the gov't always collects around 17-18% of GDP (economic activity) Whether or not it is fair or not is a matter of opinion. But history shows that when rates increase substantially people either declare less income our reduce activity.
As of now the actual portion of the tax burden paid (meaning revenues received by the treasury from personal income taxes) breaks down as follows as per 2008 tax receipts. These #'s don't move much from year-year.
The top 0.1% of earners paid 16.4% of all income tax revenue collected.
The top 1% of earners paid 38.02% of all income tax revenue collected.
The top 10% of earners paid 69.94% of all income tax revenue collected.
The top 25% of earners paid 86.34% of all income tax revenue collected.
The top 50% of earners paid 97.30% of all income tax revenue collected.
Essentially the bottom 50% of earners paid about 3% of income tax revenue.
This means that the top 0.1% of earners already pay more than the bottom 80% of earners.
What is or isn't fair is up to each person's opinion. However in my view this is already quite lopsided.