Here's the deal... you are being overly paranoid.

I regularly log into accounts of several friends and have since I was in Beta 3 of WoW. The whole don't share accounts isn't so they can rampantly ban people, it's so they can easily tell you to fsck off when you cry that your BFF you met online and have never met in person steals all your phat lewt.

Don't cheat, don't exploit, don't be a dick to people, and ensure that anyone you give your account info to isn't going to be a jackass as well and you'll be fine.

Besides, in a world of laptops, cyber cafes, hotspots and VPN Tunnels, banning by IP address would be stupid. Blizzard would be so busy banning legitimate paying customers they wouldn't have time to post their half billion in profits from last year.