Thierry,
This has always been a considerable puzzle, and the source of some amusement,
to me. It is a clear design bug in Rscript because _code that would
otherwise work_ will not if it happens to rely on library(methods). S4 comes
to mind. This has also been answered at least a few times on SO.
Now, 'r' from littler always did this differently and lods methods, just like
R itself does. And 'r' still starts (way) faster as I once documented in a
blog post.
If someone from R Core reads this and nods silently: please put this to rest
and make sure Rscript also loads the methods package.
Dirk