I thought WiFi Sync activity was limited to when you're on a power source (or at least had that option)?
You can most certainly sync via Wi-Fi without being on power. I do this every morning. Of course, the battery is not being charged, and I believe that is the reason Apple cautions users to be on power. If, for example, your battery drained completely while attempt a Wi-Fi sync, well you can imaging the results.
When I get home from work, I normally sit my iPhone (not connected to a power source of my iMac) down on my computer desk and forget about it for a while. I noticed that in iTunes, my iPhone always shows up and I can sync, backup, etc... all wirelessly. This is really nice and I think they way Apple envisioned it. As long as your battery has enough charge to complete the desired task, there is really no need to connect it to a power source. Of course, the speed of wireless syncing is not quite that of wired syncing, but there are always trade-offs when using one technology over another.