Proposed idea: bitcoins disappear from an address over time (pre-defined "prime rate" by the community). This will incentivize people to do transactions or "hack the system" by running software that moves coins every so often between addresses. The main bitcoin client should provide this functionality out of the box, so it will incentivize people to run the software (i.e. good thing to help validate transactions).
What to do with diminishing supply? Open for discussion. Bitcoins could be re-distributed as bonus to those confirming transactions, or to miners to keep mining. I like the earlier.
