A trade secret is just that, a secret. For parties unrelated to the holder of the secret, once it's no longer a secret, it's not a secret, and the former holder of the secret has no protection at all. In other words, if you're not, say, a BSAFE licensee, you are free to use the alleged RC4 algorithm.
This was my understanding *before* the recent jury decision in the Microsoft vs Stac Electronics countersuit. When Stac sued Microsoft for infringing their patents on disk compression, Microsoft countersued Stac for trade secret infringement for having reverse-engineered some hidden system calls in MS-DOS. Not only did the jury uphold Stac's bogus software patent, but they also found in favor of Microsoft on their ridiculous trade secret accusation! Needless to say, this creates a very troubling precedent. Now you can now apparently infringe a trade secret merely by examining fully public information (e.g., commercially available object code.) Phil