Post this in another thread a while ago:
During the Mumble session on Friday Sept 5, 2014 I asked BM about this and here is a summary of his thoughts:
1) Any action on the account counts, such as simply logging in to collect your interest. This is much like what banks do today with accounts that become dormant over a period of time. Also, since the platform supports an API it is possible for 3rd party software to provide a service of tending to your funds to make sure they see some activity.
2) The 5% inactivity fee provides a valuable service to the ecosystem. With most crypto currency implementations when a private key is "lost", i.e. the owner has lost access to their private keys, or very small amounts (dust) is left languishing as unspent outputs, these funds are lost forever. The network doesn't know what percentage of supply accounts for "lost" funds. With the 5% inactivity fee these funds will eventually get merged back into the network to rejoin supply.
3) As stated above this is still in the very very early discussion process. There is 10 - 11 months left to figure it out.