This might be related to some other problems I've been having for a while with tyrlight. Here's just sunlight, no dirt:
http://lunaran.com/pics/fitz0063.jpg
Opposite corner from my previous screenshot. Note that those blocks are not floating.
I would really like to say when I started seeing this kind of thing, because I know it happened after I tried to get the latest compilers six or eight months ago, but I've tried to reproduce the collection I had before that and I still get those gaps and bleeds, so I can't actually prove it's a change in some compiler that's done it.
eric, what are you BSPing with? Do you think it matters? Utter conjecture here, but could some subtle difference in point quantization or double rounding in the BSP phase cause the lightmap sample origins to fall on the wrong side of a face or become coincident with an edge in a microscopically unhealthy way?