If you are still within warranty, you can just make an appointment an go in to have it fixed or replaced.
Here are somethings that may help you get it serviced:
Source: http://support.apple.com/kb/ts1827
Tip: To isolate an issue related to a portion of the Multi-Touch display, follow these steps:
- Open the Maps application and double-tap each corner, then the center, to verify that it zooms in to each section.
- If a specific area is affected in another application, note the location and try to verify touch functionality for that location in the Maps application.
On iPhone and iPod touch, you can also use Calculator:
- Open the calculator app to test all but the top row of the screen. Press each button on the calculator and verify activity.
- To test the area at the top of the screen, rotate the device to change to the scientific calculator and press the buttons in the left or right two rows.
When speaking with a Mac Genius or Service Provider, you can use this information to assist with evaluation of your issue.
Locate a Service location: https://locate.apple.com (choose your country in the top right)
Call Apple for Mail-in Service (if available): http://support.apple.com/kb/he57
Set up service on your own or check options: https://selfsolve.apple.com
Questions about service and potential costs: http://www.apple.com/support/service/ (Choose your country after selecting your product)