Soon after the Cylon attack and the Galactica's escape from the destruction of the Twelve Colonies, Gaius Baltar says he can build a Cylon detector and works on it for several episodes. However, it is never completed (it's also never clear if he could really build one or if Six was just conning him into thinking he could) and, after a while, Adama and others stop asking him about it.
What happened? Was there a reason the interest in the detector just died off? Did Baltar admit he couldn't make one? Or did they decide that it wasn't worth sacrificing another nuclear warhead to make one?