I have a Nexus 7 running CyanogenMod 10.1. For some reason I cannot reach it from my laptop on the same wifi network. I cannot ping in either direction. In the past i have been only able to ping one way.
Just tried out on my HTC Sensation and haven't the same issue. Also the Nexus and HTC cant reach each other either.
I have had similar problems with other Android devices. In the past I was developing an app and I could not establish a socket connection from the Android device to my computer. Once I was able to successfully ping the laptop from my device everything worked.
Don't know if it is clear this isn't persistent. In the past I fiddled around and they eventually talked but there was no consistent solution.
I have observed this on 2 different wifi networks.