I have seen weirdness in version 10...sometimes, simply saving the project, closing arcmap, restarting allows me to fix this "issue" Other times it doesn't let me. Also, if you want to use a field for 2 parameters, it doesn't allow you to use the same feature layer (or layer, or featureclass). There appears to be no rhyme or reason to this and it is frustrating since the multiple input option isn't an option if you want to use one field for one parameter and another field for another. I would put this down to a bug, but I don't have the time at present to document a complete workflow to document the process...it is on my summer list of things to do.