First, I will confess that I am older than many brides here. As such, I realize that my exposure to wedding etiquette is different (perhaps out of date) than most of yours. That said, I am curious about the etiquette of inviting "plus ones."
At the time of my first wedding (1992), the rules of etiquette dictated that each guest involved in a significant relationship should receive an invitation addressed to them and their SO (defined as the person they'd been dating for a year at the time of the wedding, or someone they're living with or engaged to). Random "plus ones" were never invited (as in Friend + Unknown Guest).
Fast forward to 2011, it seems this has morphed to sending an invitation to anyone a guest is dating -- no matter how new the relationship might be. Is that truly the deal? And, is the guideline a bit looser for members of the bridal party? In other words, if a GM has been dating someone for 6 months when the wedding rolls around, should that person be invited? What about random plus ones these days?