Does anybody else that is having this problem sync with iTunes over Wi-Fi? If so, you might try turning it off and see if that makes any difference.
I noticed that I didn't get the 'No SIM card installed' alert at all when I wasn't on my home wireless network, such as visiting parents or friends, and that seemed to be a bit curious so I thought it may be something to do with the devices that are attached to the wireless network at home.
It occurred to me that it might be iTunes related because the alert was guaranteed to appear the moment I opened iTunes on my MacBook, which is fairly rarely these days because I use the iPad as my playback device and download stuff straight to it or use iTunes Match. The Mac also typically stays at the house, and when I take it somewhere else I usually don't open iTunes.
In the past couple of days I have kept half an eye on it, and it does seem that whenever iTunes is opened, or left open, the alert keeps appearing, sometimes a couple of times a minute, other times there's a gap of minutes or a couple of hours. And when the Mac is asleep it doesn't happen. So as an experiment I've turned off Wi-Fi sync, left iTunes running, quit it, opened it, left it running again, and so far I haven't had the alert at all.
I'm running iTunes 12 at this point, I can't remember whether it also did it with 11, but as there was a gap of a few weeks between iOS8 launch and iTunes 12 launch I suppose it must've done it with 11 too. Maybe I'm just in a period where the iPad has decided not to show the alert, but I thought I'd share my experience just in case it has actually worked.
I can live without Wi-Fi sync if I must, most of the content comes from iCloud anyway, and when I do want to sync and backup it is no real hardship to plug in a cable and it'll be a faster transfer too.
Doesn't stop it from having been annoying though.