Now after a good half hour of Googling, I found out that the Tab can take screenshots by holding down the back button then hitting the power button. But I wasn't even near the power button when it happened, so that couldn't be it. I also found that in the leaked Froyo builds for the Epic 4G, a few people had the same thing with a ScreenCaptureService in their running applications. Someone mentioned you had to hold down the back button then hit home to capture, but I tried pretty much all the methods under the sun and to no avail.
Has anyone else stumbled upon this and/or found out what the key combo is?