I came from an iPhone 3g two weeks ago. Both my brother and my gf have the iPhone 4, and until 3 weeks ago I thought I would be getting the iPhone 4 as well.
I am and have been on a family plan with 2 other people (but now 1 as of a month or so ago) and we still had our unlimited data. But my brother and I downgraded to the $15 package. So be sure to keep yours if you want to!
I'm not too familiar with rooting/upgrading as I am still stock pretty much. But after official Froyo I plan on rooting.
A lot of your concerns are about your plan and data, yes? That shouldn't really be changed especially on a family plan.
And the front facing camera...my gf has only used it when my brother wanted to show it off to some friends, they both say that although it is a BIG feature (for advertising the phone) they barely use it.
Hope that helps you in your final decision (even though that was pretty much one sided..but look where you are