it spams to stdout:
GetNextWorkRequired RETARGET
nTargetTimespan = %lld nActualTimespan = %lld
Before: %08x %s
After: %08x %s
SetBestChain: new best=%s height=%d work=%s date=%s
I thought this was funny

but debug.log has more useful lines:
SetBestChain: new best=8a09f01db6b050792739 height=60313 work=81439733234730 date=25.01.2014 20:39:29