Thanks for putting together the arcgisbinding package, it is very useful.
It seems that when converting a spatial dataframe to a file geodatabase, NAs in R are converted to -1 in the file geodatabase. This is a problem on many different levels as -1 may represent a real value that can't be removed if real -1 values are also present. ArcGIS does not appear to use NAs, but they do use the Null value as equivalent. While these are slightly different things in R, it would be much better to convert NAs to real Nulls in the file geodatabase than a -1 which is problematic at best. Can this be fixed in an update to this package?