I'm basically ignoring every RSVP that hasn't booked, until I hear from them or our TA that they are booked, paid, confirmed, etc. I think a gently worded evite reminder wouldn't be out of line, depending on how you word it. I actually had a couple book last week (after they got their bonuses from work), and another couple is booking this week, so maybe it isn't that late? I wouldn't wait this long to plan a trip for myself, but I suppose some people aren't OCD like me :-)