Splitting the Difference
These Mac vs. PC parody advertisements are designed to encourage voters to support marriage equality and vote against California's Proposition 8, which would bar gay marriage. Of course, given the popularity of this corporate meme, its possible to imagine a parody that takes the pro-8 side.
(Thanks to Ava Arndt for the link.)
Update: As Henry Jenkins points out, there is also an Obama Mac vs. PC parody.