Basically they realigned the 800Mhz public service band after the analog cell phones left the band. Because the frequency ranges are now different the older scanners can't deal with it correctly.
There is a whole article on rebanding in the wiki section...
Not exactly. The analog cell phone service has nothing to do with the rebanding efforts. The cell phone frequencies for analog phones use the same band as the digital phones use so when the FCC ruled that the cell phone companies no longer were
required to keep their analog systems operational, most simply added digital capacity using the old analog frequencies.
What the issue was is due to the Nextel ESMR frequencies are in the Business/Industrial part of the normal 800 MHz band and was causing serious interference issues with public safety and other 800 MHz band users. While the Nextel device may look like a cell phone, it works differently and uses different frequencies.
What I do agree with is there's a good writeup on rebanding in the wiki here -->
Rebanding - The RadioReference Wiki