I have come across the term 'conditioned phenomena'; what is meant by the qualifier 'conditioned'; are all phenomena conditioned, or are some phenomena unconditioned?
All things are conditioned (matter, feelings, perceptions, thoughts and consciousness itself). Conditioned in that their arising, form and duration are influenced by other things. Only nibbana (nirvana) is unborn, unaging and unconditioned in the original Buddhist teaching. It is because such a state exists that escape is possible from all that is born, changing and conditioned (which is ultimately unsatisfactory and the cause of suffering anyway). See Udana text, sutta 3 for more.