As far as I can tell, when using polygonal lasso tool, there is no way to undo the last point, making it SUCK when trying to cut a picture, navigate with two fingers and accidentally click a new point way across the image. You have to complete the image, then use the Lasso plus mode to include those points or remove them. As far as I know, this is the case with the tablet ipad version as well. Even when pressing the undo or back buttons in the app, it ignores the current lasso and instead undoes the last major change done to the image. FRUSTRATING as this is the main tool I use in the desktop program... almost makes it pointless in the mobile version!
In PS Touch you need to close the polygon selection first before you can undo the selection. As far as I know in Photoshop Desktop the Undo/Redo is not possible while working with the Lasso tool.