I like the windows phone 7 designer but the copy and paste of textbox is irritating. It just keep paste the textbox at the very top. It would be much more productive that it paste below the previous textbox.
So would it be possible to change this behavior by hooking this with some IDE API ?
Not sure about hooking into the IDE, but you might find it easier to just use Blend for the UI aspects. For example, if you have a vertical stackpanel, you can copy and paste and have each control appear below each other in the correct position.
If you are using the visual designer, rather than using copy and paste, you can drag a new control from the toolbox and drop it precisely where on the form you want it.
The IDE isn't arbitrarily dropping the control at the top of the screen, it places it at 0,0. You could argue that this is as good a place to put it as any. In a true copy and paste, the physical location should be exactly the same but that would be confusing.
I would expect it to be quite uncommon to place one textbox directly under another without a textblock or somethign else in between. In any case it would be necessary to change the position from where it was copied. At least with the current behaviour you'll always know where the new control is.