Can I extend my warranty again

Your warranty covers the original purchase date, not any replacement date. So you either have one year or three years from the original date you got the iPad.

http://images.apple.com/legal/applecare/docs/081811_APP_iPad_English_NA_v1.3.pdf (para 3.1)

Matt

its actually one year against manufacturer's defects or two years

http://www.apple.com/support/products/ipad.html

and you can add that second year any time in the first year. You gain nothing by waiting because it's not a year from the date of purchase of the warranty, it's a year from the date of purchase of the device.

buy it Jan 1 2010, you have a factory warranty against manufacturer's defects until jan 1 2011, buy the Apple Care and it goes to jan 1 2012.