Because the ultra-wealthy derive a large proportion of their income from capital gains, 2007 was a very good year. In October both the S&P 500 and the DJIA hit all-time highs. Combined with very low tax rates, a lot of the 400 (and many others) decided this was a dandy time to book some profits.
The cut-off and average may prove to have actually declined in 2008 and 2009.
From the IRS:
Shown below are four tables from the Statistics of Income Division which contain information from the top 400 individual tax returns for each of Tax Years 1992 through 2007. These data are based on the returns with the largest Adjusted Gross Income reported each specific year shown and do not necessarily reflect the same taxpayers over time. Consequently, tables 1-3 should be used in conjunction with Table 4, which presents the number of times an individual return appeared among the 400 largest adjusted gross incomes over the 16-year period....The tables go over salaries and wages, taxable interest, dividends, capital gains, net business income and much more.
By the way, the average income reported by the top 400 in tax year 2007 was $344,759,000.
Hear is the 13 page PDF.
The IRS also publishes some statistical analysis on estate taxes. The most recent year available is 2009:
...Highlights of the Data
• Due primarily to increases in the filing threshold, the number of estate tax returns filed decreased from more than 108,000 in 2001 to fewer than 34,000 in 2009.
• For Filing Year 2009, estates with gross assets above the filing threshold reported over $194 billion in assets.
• Almost 58 percent of 2009 estate tax decedents were male. Just under half of all decedents were married, while another 38 percent were widowed. Only 13 percent of decedents were single, divorced, or separated.
• Over 97 percent of the estates of married decedents, and 43 percent of estates overall, reported deductions for marital bequests, for a total of $62 billion. Only 10 percent of estates with a marital bequest owed estate tax.
• About 19 percent of estates claimed a charitable bequest deduction, for a total of $16 billion. Estates with $20 million or more in gross estate accounted for over 58 percent of this total, despite representing only 3 percent of filers.
• After accounting for marital and charitable bequests, as well expenses and debts of the estate, less than half of the estates filing in 2009 owed estate tax. The combined estate tax obligation of these estates was nearly $21 billion
Here is the one page PDF.