Here is some info
1. When you go out of the country the Airlines has to take your existing I-94.
2. When you return the Airlines are supposed to take the second sheet of the AP (the one without your photo), but in my experience, they did not. Gulf Air in Mumbai and Muscat and American Airlines in Paris just looked at the AP and returned it to me without taking any sheets from the AP. They did not even make copies for themselves. They did not ask me for copies.
3. At the port of entry (Dallas), I was standing in the line of Foreign Nationals. When the "US Citizens" line became empty I was asked to go there. You need to fill in a new I-94 when you enter the country. Leave the Visa fields blank. Fill in the A# from your I-485 receipt on the back of the I-94 form (there is a field for that). After initial processing at the Immigration Counter, I was asked to go into the INS room. There the officer stamped the Original AP (the one with the photo) and returned it to me (multiple entry AP).
4. I entered the USA on Jan 13, 2002. My I-485 was approved on Jan 9, 2002. I did not know about that. The IIO did not tell me about that. No other documents or questions asked.
I hope this is clear enough.