11. A portable multifunction device, comprising: a touch screen display; one or more processors; and memory, wherein one or more programs are stored in the memory and configured to be executed by the one or more processors, the one or more programs including instructions for: displaying at least a portion of an electronic document that includes a first region; detecting a gesture on the touch-sensitive surface, wherein the gesture comprises an initial portion comprising touchdown of a contact on the touch-sensitive surface, and a subsequent portion after the touchdown; determining whether the initial portion of the gesture is detected at a location corresponding to the first region of the electronic document; in response to determining that the initial portion of the gesture is detected at a location corresponding to the first region of the electronic document, selecting a first dynamic disambiguation threshold used to determine whether to perform navigation or annotation; in response to detecting movement of the contact: in accordance with a determination that the initial portion of the gesture is detected at the first region of the electronic document and that the subsequent portion of the gesture satisfies the first dynamic disambiguation threshold, performing a navigation operation on the electronic document; and in accordance with a determination that the initial portion of the gesture is detected at the first region of the electronic document and that the subsequent portion of the gesture does not satisfy the first dynamic disambiguation threshold, performing an annotation operation on the electronic document.
11. A portable multifunction device, comprising: a touch screen display; one or more processors; and memory, wherein one or more programs are stored in the memory and configured to be executed by the one or more processors, the one or more programs including instructions for: displaying at least a portion of an electronic document that includes a first region; detecting a gesture on the touch-sensitive surface, wherein the gesture comprises an initial portion comprising touchdown of a contact on the touch-sensitive surface, and a subsequent portion after the touchdown; determining whether the initial portion of the gesture is detected at a location corresponding to the first region of the electronic document; in response to determining that the initial portion of the gesture is detected at a location corresponding to the first region of the electronic document, selecting a first dynamic disambiguation threshold used to determine whether to perform navigation or annotation; in response to detecting movement of the contact: in accordance with a determination that the initial portion of the gesture is detected at the first region of the electronic document and that the subsequent portion of the gesture satisfies the first dynamic disambiguation threshold, performing a navigation operation on the electronic document; and in accordance with a determination that the initial portion of the gesture is detected at the first region of the electronic document and that the subsequent portion of the gesture does not satisfy the first dynamic disambiguation threshold, performing an annotation operation on the electronic document. 18. The device of claim 11 , the one or more programs further including instructions for: in response to determining that the initial portion of the gesture is detected at a location corresponding to a second region of the electronic document, selecting a second dynamic disambiguation threshold used to determine whether to perform navigation or annotation.