See the very narrow band of wear? A sure sign the pads are too hard to wear to match the shape of the new rim. My bad. No problem on the rear brakes, as I replaced the pads when I got the new back wheel.
See how the 4 rubber blocks are bridged together on the old, worn pad? When the new one gets ~ 2.5mm of rubber worn off, it will uncover the same bridging. This is a built-in wear indicator and tells you it's time for new pads. You will also notice the barrel adjusters on your brakes will have been screwed all the way up to take up the slack in the cable as the pads wear up to 6mm.
You can also see there is some small amount of pitting. This happens when a small stone gets stuck between your pad and rim, and it gouges the rim when it happens. If you toe your brakes you will always find this pitting on the back block of the pad because the back part contacts the rim first, catches, and sometimes holds pebbles when you apply your brakes.
You can see the pitting clearly on the last block. The dark band right on the edge of the pad is the only surface that was contacting the new rim's braking surface. This happens when new wheels have a braking surface that is angled a little differently than the old wheels, AND, the old pads are too hard to wear into the new shape.
When your rims look like this, its a good idea to scrub them lightly with Ivory bar soap (it contains a small amount of wax) and an old ScotchBrite pad to get the grit and old pad's melted rubber off the rim. (don't use a new ScotchBrite pad, it will remove too much of the rim's braking surface) Melted rubber stuck to the rim makes the brakes stick in those places, and will cause uneven braking, which is guaranteed to created skidding.
The moral of this story is simple. When you get new wheels, install new brake pads.
Note to Self: Read own blog!