I rebooted the Tab, and then realized that it wasn't online either, claiming that it was in airplane mode. Couldn't seem to change that. Searching for "stuck in airplane mode" revealed a few threads saying to remove the SD card and reboot, which did nothing for me, and a few threads saying to reflash the modem.
So, I followed the instructions in http://forum.xda-developers.com/show....php?t=1275031 and it hung up on the modem step. For like an hour. I finally gave up, rebooted the phone, and tried again. This time, it hung up at Modem again, I walked away after half an hour, and when I came back, it was sitting at the phone-exclamation-point-computer display on the phone, and the computer indicating failure.
Ever since then, no matter what buttons I mash, I can't make it enumerate as a USB device of any sort. Half the time I can't even make it turn on; it'll show the exclamation-point display only when plugged into the charger.
EXCEPT ONCE. Odin saw it, I smacked "Start", and Odin immediately failed. If I can get it to do that again, I'll jot down the exact error message.
So, right now: It's got enough sense to show the phone-exclamation-point-computer (does this string of hieroglyphs have a name?), but it won't show up on USB. What next?