Hi Bobert
Found some data to answer your question by surfing the IRS and Census Bureau sites - sorry but I guess I am not clever enough to make the links blue - maybe another poster can help.
Anyhow - here is what I came up with for 2001:

Percentage of income earners versus percentage of total tax paid:
Top 1 % paid 36% of taxes
2-5%         19%
6-10%       11%
11-25%       17%
Total       83% thus paid by the top quarter of income earners

Interestingly that leaves the bottom 75% of income earners contributing only 17% of revenue.

Hope this helps

MarkS