I've written a ton lately about best practices for relationship-building and I'm well aware that there's more to life and business than relationships (although, maybe, not that much more). Nevertheless, I wanted to use my tiny soapbox to reiterate and reinforce a message that has been delivered by others. Namely: When attempting to make an introduction you must confirm that both parties want to be introduced before doing so.
Fred Wilson blogged about double opt-in intros in 2009 and his message has been picked up by other pros at least a few times and yet even I, a non-big-shot, still sometimes get blind introduced by people who should know better.
Two bad reasons not to follow this policy
The only two reasons I can imagine someone electing not to ask their network to opt-in are (1) laziness and (2) fear of being rejected. Both of these are bad reasons.
The opt-in step is not an addendum to the introduction process. It is the introduction process.
With respect to laziness: The offer to make an introduction is both (i) an offer to share you network and vouch for someone and (ii) to expend the energy required to get the introduction off on the right foot. If you're unwilling or unable to do both parts then you shouldn't offer to make an introduction in the first place.
Sometimes people will opt-out and that is okay.
With respect to fear of being rejected: I think what might happen is that in the heat of the moment people make promises and realize later on that they might not deliver, that one of the to-be-introduced parties opts-out. It's happened to me that I've offered to make an introduction, asked someone to opt-in who decided to opt-out. There's usually a good reason - often variations on "Now's not a good time - and I've always felt a bit sheepish but, ultimately, totally fine with having to go back to to the original person and say "Sorry, they're not up for a chat". Much better that you get to relay that message than you make the intro and the busy person replies-all and says "Sorry. Can't."