Glad this was brought up.
Transaction expiration is something that I have wanted to see happen in bitcoin for a long, long time. The user experience of unconfirming transactions setting around in limbo is just horrible. Bitcoin software by necessity has gotten better about attaching fees so this sort of behavior is uncommon, but that does not eliminate the problem.
Of course, we cannot presume that a transaction will truly disappear -- The Internet Never Forgets -- but given a bit of mempool adjusting, we can achieve the next best thing: the majority of the network "forgets" the transaction and becomes willing to relay a respend of some or all of the inputs. This uses existing client logic where the client must rebroadcast a transaction until it is confirmed.
In general, if a transaction has not made it into a block within 144*X blocks, there is _some_ reason it is getting rejected by the miners.