Doesn a fully trusted user ID on a key prevent warning messages when encrypting to other IDs on the key?
No. That was actually a key validity bug in GnuPG 1.2.1 and earlier versions. As part of the development of GnuPG 1.2.2, a bug was discovered in the key validation code. This bug causes keys with more than one user ID to give all user IDs on the key the amount of validity given to the most-valid key. The bug has been fixed in GnuPG release 1.2.2, and upgrading is the recommended fix for this problem. More information and a patch for a some pre-1.2.2 versions of GnuPG can be found at:
No. That was actually a key validity bug in GnuPG 1.2.1 and earlier versions. As part of the development of GnuPG 1.2.2, a bug was discovered in the key validation code. This bug causes keys with more than one user ID to give all user IDs on the key the amount of validity given to the most-valid key. The bug has been fixed in GnuPG release 1.2.2, and upgrading is the recommended fix for this problem.