UPDATE: Such an IDIOT!! I spent about two hours on the phone with Apple Care trying to figure out why my iTunes would work fine, but I could not connect to the iTunes Store (which meant I could not stream any of the movies I own onto my TV via my Apple TV). We went through the whole drill: I turned off the iMac's firewall. No help. I reinstalled iTunes. No help. I reinstalled the entire OS. No help. I went in and deleted all the caches. No help.
What was really strange is, it would work if I started up in "safe" mode, but not in regular mode. Also, my MacBook Pro could connect with no problems. I tried disabling my wireless and connecting via ethernet directly to the cable modem. No help! Just when I'd pulled out most of my hair, I had one of those forehead smacking moments (smack! I coulda had a V8!!) That's when It dawned on me that I had installed Norton 360 a while back, which also has a firewall! I went into the Norton Firewall and wouldn't you know it - the iTunes Program was in the list of "blocked" applications! (no idea how it got there, but I must have clicked on a popup and didn't realize what it was or something).
Sometimes it just doesn't pay to get out of bed!