How long have you had this phone? How long has it been this way? Have you tried having someone else swipe their finger just to eliminate the possibility of the problem being in your swipe?
I would take it in to your closest Apple store and have them look at it. If there is, indeed, a problem, they probably will replace it on-the-spot.
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iPhone 3G (16GB, white, v3.0 firmware, jailbroken via redsn0w)
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