I wanted to ask everyone to make sure it isn't WAI before I report it, but my bro has a longsword doublestrike monk build. He got Improved Martial Arts at his 21st lvl and bam...nothing. It doesn't do anything to his +[W], and here is what the wiki says:
Improved Martial Arts +1(base damage) with unarmed. +0.5(base damage) with other ki weapons
Longswords are usually not considered ki weapons unless you have the feat:
Whirling Steel Strike You treat longswords as if they were monk weapons, remaining centered when you wield them.
So from what I gather, this should work.