24. A portable electronic device, comprising: a touch screen display; one or more processors; memory; and one or more programs stored in the memory, wherein the one or more programs are configured to be executed by the one or more processors, the one or more programs including instructions for: displaying on the touch screen display a plurality of key icons, each key icon having a fixed displayed size and an adjustable undisplayed hit region of dynamically adjustable size; receiving a sequence of individual touch points input by a user on the touch screen display, wherein: each touch point is determined at lift off of a contact from the touch screen display, and an image with an enlarged version of a character that will be selected as the character corresponding to an individual touch point is displayed prior to lift off of a respective contact, wherein the character image that is displayed prior to lift off is selected in accordance with the adjustable undisplayed hit regions of the displayed key icons; and processing the received individual touch points by performing operations after receiving each of the individual touch points, the operations including: forming a user-input directed graph for the sequence of individual touch points received so far; determining a character corresponding to a last received individual touch point in accordance with the adjustable undisplayed hit regions of the displayed key icons; displaying on the touch screen display a sequence of characters corresponding to the sequence of individual touch points, including the determined character; and updating a size of an adjustable undisplayed hit region for at least one of the plurality of the key icons in accordance with the sequence of individual touch points input by the user wherein the updating includes changing the size of at least one adjustable undisplayed hit region.
34. The device of claim 24 , wherein: the adjustable undisplayed hit region for each key icon comprises: a visible key area displayed on the touch screen display and a hidden hit region not displayed on the touch screen display; and determining the character corresponding to the last received individual touch point in accordance with the adjustable undisplayed hit regions of the displayed key icons comprises: if the hidden hit regions of two or more key icons overlap with a touch point position that corresponds to the last received individual touch point, then the character corresponding to the key icon in the two or more overlapping key icons with the largest adjustable undisplayed hit region is the determined character.