One obvious thing to bear in mind is that, at some point, the pools will inevitably all increase the difficulty of each share in order to reduce the work required to check all the submitted shares. So any ASIC-based mining system needs to be capable of checking hashes against difficulty levels above the minimum, either in the ASIC itself or in the processor controlling it. It looks like BTC Guild is actually making this change right now - there's a notice on their website saying they're doubling the difficulty per share. (This also requires changes to the software controlling FPGA miners.)

Edit: The other is that there's a good chance pools will eventually move to making miners compute midstates locally, again to reduce their resource usage. That's almost certainly best handled on a controller CPU rather than the main hash-computation hardware. What's more, since the rate of midstate computation is roughly proportional to the number of gigahashes/sec, mining ASICs would probably mean both of these happen sooner than they otherwise would.