1x is short for
1xRTT, which is by definition digital.
1xRTT is the air
interface (think "protocol" or "language" spoken over the airwaves) between your handset and the network. It's part of the evolution of
CDMA, the digital system that Verizon uses.
This indicates whether or not you're in a digital service area (or with verizon, a service area capable of utilizing 1X services... for example, the PTT feature of the v60p, and other digital data services). On the newer digital only phones (LG VX6000, VX7000, Samsung's, etc.), there is no "D" indicator for digital service (versus "A" for analog, which these phones don't have since they're "all digital"), only the 1X indicator.
Hope this helps :-)