In Dana Blankenhorn's article, Should we mandate open source Internet access?, it's a little misleading to compare broadband penetration in the U.S. with a list of countries which have dramatically less inhabited geographical area to service and which started with less infrastructure to upgrade or replace. Of those countries in the top 20, only France and Canada are of any significant geographical area -- and most Canadians live within 50 miles of the Canadian-U.S. border. Comparatively speaking, the number of households per linear mile of cable, twisted pair or fiber is probably dramatically lower in the U.S. than in those other 19 nations. Thus, the cost to upgrade infrastructure per household is dramatically higher in the U.S. than in any of the other 19 nations in the report.
Further, I would guess that few of the citizens in the 18 nations who are "ahead" of the U.S. in broadband penetration have available to them the number of very low cost dial-up options that are available in the U.S. (Remember, many nations charge per-minute rates for local telephone service.) If you include dial-up, I'd bet the U.S. is still near, or at the top of the list. That's not to say that dial-up is a good option -- but it is an inexpensive option. Changing habits and converting people from cheap dial-up to relatively expensive broadband service (roughly twice the cost in most markets) is no small task.
In support of Dana's point, in most places in the U.S., there is not even a choice between a Bell Operating Company and a cable company because the Bell concerns have not made the investment in infrastructure necessary for them to compete. They have abandoned those markets mainly because if they did build out, they would be forced to do as you propose -- lease their infrastructure to their competitors. Thus, forcing the issue will only make things worse.
As for the Bell companies being monopolies without competition...
When almost everyone I know (including the average Wal-Mart shopper) has a cell phone, I find it hard to believe that the Bell companies are not feeling competitive pressure -- especially in the once-lucrative (now commodity) long-distance markets.
With the rapid expansion of EDGE and EVDO in the U.S. market, and the ubiquity of GSM/GPRS and CDMA, not to mention the availability of wireless service in many public places these days, I am not too sure that focusing on one segment of the market, namely broadband/DSL service, tells the whole story.