I don't know about car insurance but you can share cell phone service with anyone you'd like <img alt="" class="inlineimg" src="http://www.mothering.com/discussions/images/smilies/smile.gif" style="border:0px solid;" title="smile">
When my ex and I split I had to jump through hoops to get him <i>off</i> the insurance, which was in my name (we weren't married). The insurance agency had to contact him and have him sign something stating that he was willing leaving the policy, that I wasn't just dropping him off.
Since STBX and I have different last names (his idea that I keep mine!), and I've been with my Allstate agent for 13 years, they'll do whatever I ask them. I think it's done by address... so as long as STBX lives here as my "roommate" it should work. We're doing the cell phone thing too.
What do you mean by "share" insurance? Do you mean be listed on the same policy as a driver for the same vehicle? I think you have to live together to do that, but you don't have to be married. You can share cell phone service with anyone but you might want to think twice about it because divorces can get nasty pretty fast and once you're locked into an agreement with a cell company you'll have to pay a termination fee to cancel before the term expires.