Can I use my phone as an audio device via Wifi? Is there any way to stream all audio from my (Windows) PC to my phone via Wifi, so that my phone acts like a sound card? I basically want to use my phone with my headphones as wireless headphones.

It looks like Remote Audio does this, but it is only for Linux.
I have experimented with streaming audio from PC to Android over Bluetooth. Recent versions of BlueZ are able to sink an A2DP stream, but the build I have on my phone (Droid running Sapphire 2.0.2) is outdated and won't do it. You may have better luck.

If that won't work, Android's media player may be able to receive one of the streaming protocols that VLC supports. Another poster may know more about this.