My husband and I both work, but I am covered under his medical. Does that make me a dependent?

No, that doesn't make you a dependent.
You can use the IRS withholding calculator to fill out your W4:
http://www.irs.gov/individuals/article/0, id=96196,00.html

Awesome thanks!

I agree that you are NOT a dependent, but that is NOT the crucial issue in your case.

You need to have enough taxes withheld to cover your joint tax liability. Since the FIRST dollar you earn is taxed at the marginal tax rate of the LAST dollar your husaband earns in 2011, you must OVER-withhold to compensate for the higher tax rate.

The BEST was to ensure this happens is for your husband to claim MARRIED with ONE allowance and for YOU to claim SINGLE with ONE allowance. It is too late in the tax year for this to have a significant effect on your tax return for 2011, but that should cover you for 2012.

If you get a BIG refund in January 2013 on your 2012 tax return, then you can adjust the withholdings accordingly.