Problem: Selection on a raster returns an error if no output raster is specified in the Raster Calculator
Typing "[inlayer] == 5" in the Raster Calculator returns the error "Failed to evaluate the calculator equation" but typing "outgrid = [inlayer] == 5" works.
This is a known issue with ArcGIS 8.1 and 8.1.2.
Solution or Workaround
Upgrade to ArcGIS 8.2.