I think that Wifi will become more widespread, but will not be everywhere (for instance on the highway or suburb areas) but i can definitely see it blanketing a large city, like New York: sit in any outdoor café, on the bus, walking down the sidewalk (pda) and have a 2.5mbps+ internet connection with a city-wide wireless plan (thru t-mobile or someone). And when youre somewhere else, like on the highway, you can use a 250kbps connection through a cell phone.
On the issue of stealing a net connection, this is prevented through only letting one ethernet address connect per account (every ethernet or wireless card has a unique built in address). Some DSL companies currently use this, although it is now become less frequent (it was normal for DSL when it first came out).