this is annoying the ***** out of me and I can't for my life figure out why it's happening. everything works fine for my iPhone 4, but my iPad 2 just refuses to show up on the iTunes sidebar unless I plug it in with a cable first. otherwise I can't get it to wirelessly sync
I had a similar issue and the trick that worked for me was to Plug my iOS device to iTunes Uncheck the "Wireless Sync" box Hit sync Now check that box Sync
I had the same problem with an iPad 1, but it eventually just resolved itself somehow. I wish I had a more specific solution to offer, but a good start would be to make sure you have the very latest updates to iTunes and iOS.