Proof of concept, needs more work to be usable. Issue comes from mouse positions in event are from previous modal call but the position during initial execute should be considered.

timed_edit_mode.py

# made in response to
# http://blender.stackexchange.com/q/46461/935

bl_info = {
    "version": (1, 0),
    "blender": (2, 75, 0),
    "author": "sambler",
    "name": "Timed edit mode",
    "description": "Use tab to go into edit mode, if tab is held and mouse is moved then show the mode pie menu." ,
    "category": "test",
}

import bpy

time_to_wait = 0.5

# we duplicate this from ui_pie_menu_official
# so that we still work if it is not enabled
class MyModeMenu(bpy.types.Menu):
    bl_label = "Mode"
    bl_idname = "OBJECT_MT_mode_menu"

    def draw(self, context):
        layout = self.layout

        pie = layout.menu_pie()
        pie.operator_enum("OBJECT_OT_mode_set", "mode")

class TimedEditMode(bpy.types.Operator):
    """Go into edit mode if tab is held down"""
    bl_idname = "wm.timed_edit_mode"
    bl_label = "Timed Edit Mode"

    _timer = None

    def modal(self, context, event):
        if event.type in {'RIGHTMOUSE', 'ESC'}:
            self.cancel(context)
            return {'CANCELLED'}

        if event.type == 'TIMER':
            move_x = abs(event.mouse_x - event.mouse_prev_x)
            move_y = abs(event.mouse_y - event.mouse_prev_y)
            move_threshold = context.user_preferences.view.pie_menu_threshold

            if move_x > move_threshold or move_y > move_threshold:
                bpy.ops.wm.call_menu_pie(name='OBJECT_MT_mode_menu')
                return {'FINISHED'}
            else:
                bpy.ops.object.mode_set(mode='EDIT', toggle=True)
                return {'FINISHED'}

        return {'PASS_THROUGH'}

    def execute(self, context):
        wm = context.window_manager
        self._timer = wm.event_timer_add(time_to_wait, context.window)
        wm.modal_handler_add(self)
        return {'RUNNING_MODAL'}

    def cancel(self, context):
        wm = context.window_manager
        wm.event_timer_remove(self._timer)

addon_keymaps = []

def register():
    bpy.utils.register_class(MyModeMenu)
    bpy.utils.register_class(TimedEditMode)

    wm = bpy.context.window_manager
    kc = wm.keyconfigs.addon
    if kc:
        km = wm.keyconfigs.addon.keymaps.new(name='Object Non-modal', space_type='EMPTY')
        kmi = km.keymap_items.new(TimedEditMode.bl_idname, 'TAB', 'PRESS')
        addon_keymaps.append((km, kmi))

def unregister():
    for km, kmi in addon_keymaps:
        km.keymap_items.remove(kmi)
    addon_keymaps.clear()

    bpy.utils.unregister_class(TimedEditMode)
    bpy.utils.unregister_class(MyModeMenu)


if __name__ == "__main__":
    register()

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Thanks