I've been using Identi.ca but too many of my contacts won't use it and I want to move to Twitter instead. Is there an app that can go through my list of subscriptions/subscribers and match those to tweeters, so I can do a bulk follow?
As Wil has alluded to, that would require that your Identi.ca followers use the same handle as they do on Twitter. While this is likely, it is but by no means a certainty and thus unreliable.
The problem you present suggests that you don't follow that many people on Identi.ca. I would suggest enabling cross-posting from Identi.ca to Twitter, posting to Identi.ca that you're moving, and soliciting @-mentions on Twitter from those people so you can quickly find and follow them.
I don't know of a better solution at this time, I'm afraid.