“Do I have to pay any Capital Gains tax?”---->I guess so; as a US person( a US resident), you are subject to US taxes on your world wide income and US source income. You pay tax on your LTCG to Indian taxing authority(ies) in India and you can claim your LTCG tax paid in India on your US federal return as long as your US tax rate is higher than that in India by filing IRS form 1116. I guess you can’t claim your Indian LTCG tax on your US state return, BUT only on your federal return. This means that your LTCG is subject to double taxation to both India and your US state. For Foreign Currency Conversion between Indian Rupee and $ US, you may need to use weighted average currency value.You report your foreign tax credit on 1040 line 8, other taxes, I guess or on form 1040 line 47.REMEBER: foreign taxes that are not eligible for the foreign tax credit may be eligible for the foreign tax deduction.
“If not taxable, do I have to mention any reference of the same in my 2010 returns, which quite frankly, I did not mention.”---->As said above, you need to file FORM 1116 with the IRS.
Please visit the IRS Website; http://www.irs.gov/pub/irs-trty/india.pdf