Yes, iTunes has to be opened. But you don't have to plug in your iPhone. I'm syncing it without it being plugged.
Just make sure that both your computer and iPhone are on the same wifi network, and make sure you check "sync with this iPhone over wifi" on the Summary page in iTunes. That will let your iPhone show up under "devices".
To start syncing from iTunes, just click on sync like u normally would.
To start syncing from your iPhone, just go to Settings->General->iTunes Wi-Fi Sync->Sync Now.
and voila
p.s. mine has been working like a charm!