Thanks to all that responded! Here's an update.
I have a very close, family friend who builds homes and is an all-around handy-man (and who I completely trust!) He came over to look at the problem and didn't see any immediate threat. The said that the beams were not over-extended and that the span was okay. He said that part of the floor was in a very high-traffic area (it is) and that he'd check to see if any of the beams were cracking or rotting. So, he worked from underneath the floor, from the living room below. He didn't find anything wrong with the existing beams. He reinforced them with additional wood beams, every 16 inches or so. Fortunately, I was out of town on business when he was doing all of this. :-) When I got home last week, the ceiling was mostly patched up. He's coming by again this week to finish mudding it. He told my husband that you could put a waterbed up there now, it's so sturdy. When I got home I made my husband walk all over the floor and the horrible creaking sound is indeed gone. It still creaks a tiny bit in a few spots, but I'm convinced that's just a couple loose nails on the subflooring.
So, case closed I hope. I feel so much more comfortable sleeping up there and I don't freak out when my husband walks upstairs and I can hear it from below. Now, I'm being neurotic about other things in the house!
Piffin, interesting that you should ask about my husband's size. He's about 6'3", 260 poundsand no, he did not play a green guy on TV. BUT, and I'm not kidding, I did date a guy for a couple years who looked JUST like Lou Ferigno. People would stop him on the street to comment about the resemblence! Funny...