I am almost too scared to ask why it doesn't work but I am sure as hell it caused some major grief for people in the past.
The reason is that the ADC is a bit different, and a bit more complicated, than on other chips. On the 18F2431 there are two sample and hold circuits so it can sample two analog voltages at the exact same time (and then convert them one after the other). It can also be setup to run continously in various configurations and put the results into a 4 word FIFO, an interrupt can be generated at various "fill levels" of the FIFO.

/Henrik.