Update: I decided against going to Arlington in person. Simply can't afford to take a day to do that. So I got in touch with Senator George Allen's office instead. A very nice and competent-sounding lady by the name of Emily instructed me to send a written information request to her, and promised to take it up with the Arlington Office. She also told me there was a way to check with USPS on whether Postal Money Orders have been cashed. I did just that, and found out that all of mine were cashed on Dec. 9, 2003 (my application to Arlington went out on Nov. 25).
Emily also told me that, if you send a "one-stop AOS package" (that is, concurrently file the I-130, I-485, I-765, etc.), the Arlington office indeed may NOT send you a receipt.
Finally, she confirmed the rumor that the Arlington office will soon be moving to a new location in Merrifield, VA (only a few miles from their current location). The new office building is going to be "much nicer" than the current one. However, the move will not be happening until this spring, perhaps around March or April.