First of all $65,100 is NOT the upper limit of adjusted gross income for a married couple for the application of the 0% capital gains tax rate --- $65,100 is the upper limit of TAXABLE income for a married couple for the application of the 0% capital gains tax rate.
Taxable income is calculated as (adjusted gross income) - (standard or itemized deduction) - (personal exemptions).
To qualify for this zero percent capital gains rate - the gains have to be long-term and the taxpayer has to be in 15% tax brackets or below for regular income. $65,100 is an upper level of the 15% tax bracket for a married couple. That zero percent capital gains rate - is used for 2008 and 2009 instead of 5% rate used before. There is no 5% capital gain rate in 2008 and 2009.
You need to consider how that capital gains would be taxes if it would be a regular income - that part falls into 15% tax bracket or below will not be taxable in 2008, but the part falls into 25% tax bracket or above will be taxed at 15%.
Consider this example - your total TAXABLE income in 2008 is $60,000 regular income plus $40,000 long term capital gain - total $100,000.
Without capital gain - you are in 15% tax bracket (upper limit is $65,100)
Part of the capital gain - $5,100 falls into 15% regular tax bracket and as long term capital gains - will not be taxable in 2008; but another part $40,000 - $5,100 = $34,900 falls into 25% regular tax bracket and will be taxable at 15% - the max long term capital gains rate.