While tax revenue is growing far faster than the Bush administration forecast in its budget projections in February, the nation's economy isn't. What has changed isn't the size of the economy, but how the economic pie is divided. The share of national income going to corporations and the wealthiest individuals, already large, has expanded, while the share going to typical wage earners has shrunk. Because corporations and the wealthy generally pay income tax at higher rates than does the typical wage earner, that shift benefits the federal Treasury.
From the Wall Street Journal via the Tax Profs Blog.