I do not think we are reaching a limit..if anything its the other way around..LTE and wimax for example are much more efficient and can handle more simultaneous users then 3g.
Now as for the actual bandwidth sources via lines and etc, we are not in a bad situation due to hardware limitations but more that the companies being cheap and stingy. If you think about it, korea and japan use up a lot more bandwidth per user then we do in the USA..USA ranked 16th in the world.
It boils down more to economics then it is anything else....
In the future NASA plans to launch 5g with involves using a network of small satellites 110 pounds to provide bandwidth and gigabit speeds.
Edit:
here is the official press release in 2008
http://www.nasa.gov/home/hqnews/2008...s_nanosat.html
Edit 2:
Here is the whitepaper for Intel's wimax efficiency in comparison to other standards today.. it is possible to be a bit biased as Intel invests in wimax(link is below article)
http://www.wimax.com/commentary/blog...ays-intel-0315