It appears to have something to do with desktop composition. I need to run Visual Studio 6, and to make the VB6 form editor work correctly under Windows 7 I had to check the "Disable Desktop Composition" box in the Compatibility tab in the properties for that executable. When I'm running VB6 it puts windows 7 in a 'Basic' color scheme.
When I start up chrome before I run VB6, link previews work just fine. But afterwards they don't work at all, even after I've exited VB6, until I restart chrome. This also happens if I start chrome while I am running VB6.
Because it's such an obscure circumstance, I doubt this will ever be fixed so I won't hold out too much hope for a genuine solution.
Woohoo! Looks like this is fixed in chrome 29.0.1547.66.
This seems to reoccur occasionally so I'm not sure if this is actually fixed or not. It's been more frequent as of late, but I can't seem to reproduce it at the moment. I'm now on chrome 34.0.1847.116.