A junkyard bike I found had what looked like a first-gen Trek digital speedometer on it. I took it off to use on a different bike, and when I went to remove the magnet I was really impressed; the magnet was just a little circular one, but it was pressed into a slotted metal cylinder that was tapped for a little bolt.
I can see why this changed; heavy bicycle parts don't sell. Still, the vintage Trek one only _looked_ heavy - it was rock-solid, and outlasted the bike it was on.
To those with the rare-earth, and neodymium magnets: just remember that putting a super-strong magnet on means you have to watch how you park your bike.
If you park with the magnet right over the reed switch coming off the speedometer, you'll magnetize it. Once its magnetized it won't ever work right again (or it won't work at all).
This is also why you shouldn't really buy a used bike speedometer - most of the time, people take one off a bike and just put all the parts in a ziploc bag, which eventually destroys the reed switch.