Well, I traced it to a "radio problem" with a client device. It was a "Belkin F5D7330 802.11g Wireless Ethernet/Gaming Adapter" I was using to connect a network printer that has only an ethernet port to my wireless network (so I wouldn't have to run CAT5 cable to where the printer is situated).
To do this I had to remove/disable ALL my wireless devices and computers, then add them back in one by one... It was frustrating because I'd forgotten that printer was actually connected wirelessly, I was used to thinking of it as connected via an ethernet cable since I "knew" that's what the printer supported. I even unplugged my 2.4 GHz wireless phone base, which I actually believed to be the culprit. I only remembered the printer's wireless adapter after everything else was removed.
But once I unplugged the Belkin's power, boom, my wireless network was back to normal! And plugging it back in caused the problem again. So it's bye-bye Belkin.