It's really sad to see people responding to threads like these with completely wrong information. What I'm about to type out is the whole truth, AND NOTHING BUT THE TRUTH. Ignore everything else that you've read.
If you want to upgrade a line, and use the smartphone on a different line, you can remove the smartphone package from the line that you upgraded. YOU DO NOT NEED A DATA PLAN UNLESS THAT LINE IS ACTUALLY USING A SMARTPHONE. If you're currently using a smartphone with "SMARTPHONE EXLUSION," you cannot go back to not having data if you upgrade.
Example A:
Your kid is eligible for an upgrade, you're not. The kid is using a Samsung Impression and he loves the **** out of that thing. You can upgrade his line, and put your SIM into the Galaxy S II. Your kid can keep tapping away on that Impression like nothing happened. You can remove the data package from his line. Just have the rep remove it. You don't even need to supply his IMEI, they can just remove that ****. No problems. You just need to have the appropriate data plan for you line, since your SIM is in there. eg- an iPhone or BlackBerry data plan will not work with the Galaxy S II, you need "HSPA+ Smartphone 2GB."
Example B:
You can add a line to a current family plan. That line will include a data plan, but you can simply remove it and put the SIM into a cheap GoPhone, or throw it in a drawer somewhere. This WILL cost you more money in the longrun if you do not have a need for the extra line. An Early upgrade for the GSII will be $450. Adding an unnecessary extra line to your account will cost you more money than the early upgrade fee of $250 on top of the $200 sale price. At an approximate cost of $12 (9.99 add-a-line, plus taxes +fees), you're looking at $324 over the course of two years, including the $36 activation fee. This $324 is on top of the sale price of $199.99. (Activating additional lines went from $26 to $36 a few months ago.)
If you have unlimited on your current line, you can DEFINITELY keep it. There's no need to lose that. You can always get it back if you need. There's always a way to get it back.