I was using an earlier seat of VC6P and noticed this inconsistent surface utility behaviour. I just verified it in Shark FX 7. Draw three sides of a rectangle and surface extrude a channel. Fillet the two edges, then try to thicken the surface. It's refused with an ACIS error message. Fillet the profile before extruding and it thickens without complaint. It's easier to alter the radius of an edge by simple data entry than trekking through the concept explorer to replace a curve. Both VC6P and FX V7 worked as I expected when I extruded a solid from a closed profile, filleted the edges, and then shelled with open faces. I haven't tried it with V8 yet. BTW, fillet the edges of a surface channel and the rendered view changes, with faces becoming transparent depending on the angle they're viewed from.

*Same deal in V8, including the rendering glitch.