Droid 2 Global is like this too. Yes - if the phone proximity sensor is engaged (phone in your pocket/etc), the ring will show slide to answer. If proximity sensor detects no close-by surface/object - it will show button for answer and reject.
The idea is if the phone is in your pocket, to display the slide so you don't accidentally answer or reject the call while you are pulling it out. And respectfully - if you are working on the phone, or phone is on the table - to show you buttons for easier touch (so you don't have to pick the device in your hand in order to accept/reject the call.
This btw is one of the fetaures of BLUR that I like a lot!