Wow. Some thread, hey? Lemme finally add my two cents...lord knows I've been trying to reply in here for weeks.
I've toyed with the idea of a threesome for quite a while. Luckily, mandy and I have a relationship where we can share absolutely anything with each other. When I brought it up with her, we discussed it in the way we always talk about things - keeping an open mind. And while a threesome might be more 'out there' than say, some light bondage, the manner in which it is discussed remains the same. Both parties need to raise any concerns they may have and address them accordingly.
As the TFP motto goes, communication is key. Before there can be opportunity for jealousy to rear its head, all issues surrounding the idea need to be cleared. All the whats and wheres and ifs and buts need to be covered. In the case of threesomes, boundaries are important. Discussing what you're both comfortable with is very important. Your SO might not want you kissing the new partner on the mouth, or you can lick her pussy, but not her ass and so on. Obviously these boundaries need to be discussed with the 3rd party so that everyone involved knows what's allowed and what isn't.
mandy and I have discussed this all at length. We both know what we want from the experience and why we want it. We've taken what has been said in this thread into consideration, and I want to thank all for their advice and opinions.