No, it is not taxable income on your parents taxes. Scholarships/grants are taxable income if they exceed qualified education expenses, but they are taxable income to you - not your parents. Even if you are claimed as a dependent on someone else's tax return, you can still file your own tax return.
Your parents may or may not be able to claim you as a dependent on their taxes. As you are 24 years old you are no longer a qualifying child for tax purposes so they can not claim you as a qualifying child. They may be able to claim you as a qualifying relative if you meet all the criteria ( Not a qualifying child, Gross Income, Total Support,
Relationship, Joint return, Citizenship). The ones that seem most pertinent to your situation are the Gross Income test (your gross income can not exceed the personal exemption amount - $3800 for 2012), and total support (they must provide more than half your support)
Whether they can claim you or not is something you need to figure out.There is absolutely no sense in turning down the Pell grant. If it is taxable, it is taxable to you whether you are your parents' dependent or not. If you have no other income, it is likely not going to incur any taxes. But even if it did, you would still be better off with it than without it. (for example if $2000 was taxable at 10%, you would pay $200 in taxes and have $1800 left. Where is the sense in throwing away $1800 to save $200 taxes?