This problem is addressed in ArcGIS 8.1 Service Pack 1. See the Related Information section to download this pack.
VRML export for scenes has an option to use an "ElevationGrid" data type when writing out information for raster-based surfaces. This should be on by default as its use produces much smaller VRML files than the alternative. Instead, it's off by default, and use of the check box to turn it on has no effect.
This is a known issue.