Bind Variables ignoring Index on 10.2.0.4

My company sells and app that's been running atop Oracle for about 18 months now. We've run on 10.1.*, 10.2.0.1, 10.2.0.2, 10.2.0.3, all without any unexpected behavior. Recently, we had a customer install us atop 10.2.0.4 and we've seen some very unexpected behavior with regard to our queries. To whit, anything with bind variables seems to