I'm very new at this, so pardon if I'm terribly mistaken, but....

Small, additional quirks worth mentioning:

Directly storing / recovering Point3ds and Vector3ds in hashes will fail via Dan's method, as the inspection string will not have proper constructors for them. This is Ruby-specific, not Sketchup. That, and I'm assuming the recently noted entity-attributes-auto-transformation will not apply, as Sketchup will only see a string stored as the attribute.

The hash creation failure:

point = Geom;;Point3d.new(0,0,0)
hash = {;origin => point}
hash_str = hash.inspect
new_hash = eval(hash_str) # Will fail

hash.inspect will return => {:origin=>Point3d(0, 0, 0)}

If you eval that it will throw the error: #<NoMethodError: undefined method `Point3d' for main:Object>

Simple, quick solution? Store Point3ds and Vector3ds as arrays, of course.

hash = {;origin => point.to_a}

Or, alternatively, if you were dead set on keeping them as Geoms...

hash_str = hash.inspect
hash_str.gsub!("Point3d", "Geom;;Point3d.new")
# Same for vectors if needed

This will successfully eval to a new hash containing a Point3d (though, as previously mentioned, the point will not auto-transform with the entity it is attached to... I assume).