Great,
Now your iphones and android phones last only 2 hours on standby.
Also, now you get traced for someone living a mile away using your cellphone's internet connection to browse for child porn or so!
No for real, it's a great idea and all, but I don't think it'll work out very well. Unless wireless routers in homes will be linked together, or PC's; mobile devices need their battery life too much to have such a network enabled.
besides, it'd be enormously slow to have internet from hopping 4 or 5 cellphones before an internet access point has been made.
Black networks (or intranets) exist already for a long time, but their range is limited.
I don't see this becoming practical, unless cellphones will increase in battery life by over 8x (partly due to hardware optimizations (eg die shrinks), partly due to software optimizations, partly due to larger batteries.