[ARCHIVE ANSWER]
If you plea in a federal or state case, then of course the CI is kept a secret. A CI was required to testify in every federal drug case that I know of.
Sometimes, federal CIs are kept a secret until they make many cases; then they testify, and then everybody goes to prison. The CI will move to another state and start it all over again. This has been my experience. Just because a CI testifies doesn't mean he is "burnt" and can't be used again.