Last I heard, bagginsbill had filed a bug report with Sith Micro - so there may be a fix sometime around Poser 9 or 10... :)

If you're using a map which uses mid-grey to represent "no bump", as Poser 4 bump maps did, you'll get this effect. You can use a math node to subtract 0.5 from the bump map, which will return the mid-point to zero. It helps on some maps, but won't be 100% effective on them all.