The Insular Scandinavian (IS) languages, Icelandic and Faroese, exhibit variation in subject case marking, primarily pertaining to an older idiosyncratic case and an innovative case that instantiates a regular pattern. This variation occurs to a considerable degree at the level of the individual speaker. We argue that this intra-speaker variation in IS involves optionality within one grammar and not competition between two different grammars or dialects in the sense of Kroch (1989, 1994). Mainly on the basis of data from the so-called Dative Substitution (DS) in Icelandic, we show that this variation does not involve any kind of parameter, and that it is not the result of dialect contact, as the grammar competition analysis would entail.