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A microcontroller is programmable in software - you can make it run a program which interprets input signals and sets outputs. An ASIC is programmable "in hardware" - you can actually reconfigure the chip so that a particular configuration of input signals produces a particular set of output signals. This is usually done using FPGA (field-programmable gate array) or CPLD (complex programmable logic device) technologies.
The Metronome controller used in PRS-505 is implemented using an ASIC - an Actel ProASIC3 FPGA to be specific. I assume they used the same or similar approach in the iRex 1000. Metronome is faster than the older Apollo because it uses a 16-bit LCD interface to transfer image bits versus 8-bit custom protocol in Apollo. The new Epson controller should be even faster since it executes some of the processing itself instead of relying on the main CPU.
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