Floating the links to the left works. God knows why.
The live example is obviously a simplified version of what I'm actually doing. As I was applying this fix to the real thing, something else turned up.
Floating the links to the left only works if the padding is also to the left (in the example above, I have padding-left:2em; but on the real site, I used padding-right). In order to fix this, I gave the links padding-left, and then had to give the span a padding-right. Ay carumba.
I'm just going to chalk this up to IE7 sucking, and wait for it to die a creeping, unnoticed death, when IE9, 8, and 6 all have more market share.