I will keep the Eris through my 20 month upgrade period. Also, we have a family plan, so, based on the fact that you'd need to buy a Nexus One unlocked to add to a T-Mobile family plan, and because I think that Verizon will have a similar policy, that would not work for me. Lastly, I do like the ease of dealing with just one party if I am having an issue with my phone, so, even if it is available to add subsidized to an existing family plan, I'd probably still be reluctant.
When the kids are off on their own and pay for their own phones and plans? Then we'll see.