I've enabled WiFi sync for my 4S and my first gen iPad, both running iOS5. When my phone is plugged in to my computer with a USB cable, it shows up in the sidebar, but as soon as it's unplugged it goes away. The iPad stays in the sidebar until I manually eject it.
It seems that my iPad is behaving 'correctly', at least based on This kbase article.
Toggling WiFi sync when the phone is connected doesn't seem to solve it.
I've also tried the steps outlined in this article to no avail.
This was a fresh install, I didn't import a backup from a previous device. I've restored and set up as new, and the same behavior happens.
I had originally thought it was because my computer had switched to an 802.11n 5Ghz network while my phone remained on the 802.11n 2.4Ghz network, but even when they're on the same exact network I see this behavior.