Cool - but cooler with a trigger input? Add a pot to one analog input to set the trigger level - not that much more work?
You're tempting me! I may just go ahead and make a full blown scope out of it!
I'll have to add it to my list of projects "to do".
The big one I'm working on now is for a research project at the university here (my employer). I used to use Motorola 68HC11 EVBU boards for student projects, but the Arduino is SO much easier to program (C vs raw assembler) and the 328P has a lot more memory than the HC11 (the HC11 only has 512 bytes of SRAM and 512 bytes of EEPROM and no flash at all!).
For every project I had to build daughterboards ("shields" in Arduino-speak) for I/O, memory, D/A, etc... now all of that is built into the chip - which costs less and is easier to program.
Code that would have taken me weeks to do in ASM I've done in only a few days with the Arduino. Wish I knew about these a lot sooner!