I'm at the MacHaus, trying to show them what my keyboard is doing. And, of course, it is working fine now. Since it tends to need to be used a bit before the cursor starts jumping around, I'm typing here to see if I can get it so they can see what my problem is.
But except for one or two errant jumps by the keyboard, all is well.
But, it turns out, there's a crack on the top case. There was one before and Apple replaced it and the keyboard free once already because it was a design flaw. Apparently the edge on top (the casing around the screen) seems to cause that problem and so they will replace the top case, keyboard, trackpad, and the topcase for free. That might solve my problems.
And they have the parts available in town. This is looking better than I expected.
There are some pictures of the crack the first time - January 2009 - here. This time the crack is on the left side, not the right side. But if you have a crack like that on your MacBook, and it's less than five years old, it seems like they'll replace it all for free.