I had a similar experience. Everything was working fine, I was getting 51.3 kbps, and then the phone company "upgraded" the neighborhood. No more high-speed dial-up for me - I'm lucky to get 28.8, most of the time I get 26.4.
A couple of weeks after the "upgrade" the phone company sent me an ad offering to hook me up with ISDN (yeah, right!
Not really - There was a story in the Dallas Morning News last year about local phone exchanges being maxed out because more people are getting second phone lines for the PC. Dallas-Fort Worth switched to 10 digit calling because of that.
In my neighborhood our phonelines/electicity/TV cable is buried underground so the prospect of Verizon digging up yards to install a new main cable or fiber optic line (which I hear isnt 56K compatable either!) would be a huge hassle. It's cheaper for them to "pair gain" multiplex phonelines than installing more cables to handle the traffic.
I just hope more people see the benifits of a high bandwidth connection and switch over. Two of my neighbors got cable modems the next day after they came over and checked out my connection!