I would use something along the line of this:
CON(isnull([raster]), FOCALMEAN([raster], rectangle,4,4), [raster])
You can change the size of the rectangle. I am sure someone probably has a better way than this, but it generally works well for me.
Patrick
EDIT: Just realized this was posted in the 3D Analyst forums, not Spatial Analyst. My suggestions requires Spatial Analyst extension (as does the Fill tool suggested previously).
2nd edit to add syntax for ArcGIS 10: Con(IsNull("raster"),FocalStatistics("raster",NbrRectangle(4,4),"MEAN"),"raster")