I actually participated in the engagement of one of my best friend's ex's to her new boyfriend. They were visiting from out of town and the dude (who I don't really know very well) calls up and asks us to plot a way to get her up on this mountain side where he'll be waiting to propose.
I felt a little weird because she's my best bud's ex, but she's my friend too in her own right, so meh...
As for my own ex's... I would definitely feel weird if the most recent one did it now. We've only been apart for a little less than a year and our relationship was pretty serious and headed in the commitment direction before shit started falling apart.
Second most recent: meh, wouldn't bother me too much. I don't talk to her much these days and she's kinda crazy. I guess I'd feel happy for her.
HS/early college ex: For all I know she's already shot a few kids out. Saw her about a year ago and was too trashed to remember, but I know she's been in long-term relationships. I'd feel weird for like 3 seconds and then I'd go "meh, whatever."