What?s the distribution in the amount of income donated to charity by
U.S. citizens (working adults)? The answer will ideally be something
like this: 50% donate 0% of their income, 20% donate .5%, 10% donate
1%, 10% donate 2%, 5% donate 3-5%, 4.9% donate 5-10%, and .1% donate
more than 10% of their income. (Note: I'm looking for more than just
an average amount). If the researcher comes across any comparison
information for other countries, that would be welcome, too.

Here's a start with a bit of 2000 data:

http://www.artsnet.org/aaco/economics_pdfs/Chapter%20Five%20-%20Revenue%20Sources%20for%20the%20Nonprofit%20Arts%20Industry.pdf

33% of Americans used itemized deduction for their 2k tax returns.
The NCCS using some math and some analytical thinking assumes this
makes up about 86% of US giving.
Itemized deductions for giving totalled $137B.

So 33% of the people gave $137B * 86% = $118B.
   67%               gave               $ 19B.

This site doesn't indicate how much money these 33% or 67% make, so
there is no way to determine a good percentage of income that this
accounts for.  But if you want to assume that these are an average 33%
of Americans who are giving so much then here is the math:

US GDP: $12.5T
$12.5T * 33%  = $4.2T
$118B / $4.2T = 2.8% (this is what the top 33% of American givers are giving)

$12.5T * 67%  = $8.3T
$ 19B / $8.3T = $.2% (this is what the bottom 67% of American givers are giving)

Not perfect info, but it's a start.