This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
Although the intent is that the ! operator should be usable with an operand that is a class object having an explicit conversion to bool (i.e., its operand is “contextually converted to bool”), the selection of the conversion operator is done via 126.96.36.199 [over.match.oper], 12.2.3 [over.match.viable], and 12.2.4 [over.match.best], which do not make specific allowance for this special characteristic of the ! operator and thus will not select the explicit conversion function.
Notes from the June, 2014 meeting:
CWG noted that this same issue affects && and ||.