Firstly if they did do it they would not be copying Sony....Go read up on console history, you might be surprised at what you find :)
On topic!
We are what 3 day's from Microsoft's conference. Normally leaks start happening way before now so if they announce/show a new 360 unit at E3 i would be surprised. Not surprised that they announced it but surprised nobody leaked it before it was announced. We all knew about the PS3 one way before it was announced.
So personally i don't think they will announce one at E3. I do think however they will announce a price drop and a Natal Bundle. If they come out of E3 with no price drop i would be shocked. Outside of america sales of 360 are dropping badly. If they don't drop the price or announce a new model then you know something is happening behind the scenes. By keeping it as it is they are settling for 3rd place because in 12 months time the PS3 and 360 will be very close.
So if that happens i got a sneaky feeling we might be seeing the next console in the following 12 months. It would be going into the 7th year on sale so time would be about right.