As reported many times here, you cannot have an empty value in a drop down created from a predefined values table.
http://www.boonex.com/unity/forums/?action=goto&my_threads=1#topic/Drop-Down-Menu-Select-It.htm
This simultaneously screws up both the join form - because whatever is at the top will be entered as the value; and it screws up the searches, because you can only have one drop down in a search - otherwise it will take the default value of one of them - the top value. There is no way to "null" it in either place.
For some reason we have had difficulty getting Boonex to accept or understand this issue - I even heard a developer call this "a feature". This is strange, because it worked perfectly well in D6. I believe there is a ticket for this for 7.1, but this should be fixed before this. In fact, this is a critical but and D7 probably should not have been released without this working.
Rob