I am not a Wiccan, despite having noded a bit about Wicca. But on my searching path, one thing was made quite clear from the limited information available online -- the Wiccan Rede is an ethical standard, not some mystical undo command.
Every time I've seen it explained, it's not something you tack on to keep bad stuff from happening; it's something you use to determine if you're going to do the Wrong Thing. Add to that the fact I've never seen it tacked onto a spell (unlike, say So mote it be), and I think PureDoxyk's hypothesis is unfounded.
Example 1:
Jane is lonely, and so decides to cast a love spell to get Billy. Jane does so. Said love spell is a bad thing and will result in bad mojo because it takes away Billy's free will away, akin to kidnapping him.
Example 2:
Jane is lonely, and so decides to cast a spell which will encourage love to be around her. Said love spell is acceptable because it doesn't force anyone into it; it's akin to her getting a (I hate this word) makeover.
Update (2003-07-25): I have since found that there are a fair number of Wiccans who do tack on things of the sort
PureDoxyk indicates above. Though, depending on one's view of magic, that doesn't mean it won't work. It wouldn't for
PureDoxyk, though.