fivepoint, since your iMac is seemingly going to remain in the loop under all scenarios, it seems to me you might want to consider changing your design a little.
It sounds like you want to hang some USB storage off of your router (AEBS) that will contain your iTiunes content (music, TV shows, movies, etc.). All doable, but understand that depending on your network, this may introduce more frustration that it's worth.
If, for example, your network is wireless, then depending on the weakest link in the overall system, the speed and reliability of the connection could be called into question. Likewise, with a wired network, you could be running over some homemade system that leverages pre-existing in-wall coax (slow) or brand new cat. 6 cable with 10G capabilities (fast). The point is that the capability of the network comes into question.
Another option would be for you to simply hang the USB storage (although I would suggest Firewire 400 or 800 if you have such ports on your iMac) directly off of your iMac. This way, you will likely (A) increase significantly the throughput speed from the storage device to the iMac, and (B) reduce the amount of traffic required to be carried by the network.
Since most people seem to physically locate their router close to their cable/DSL modem (which for many is located in the bowels of their abode), if you hang the storage directly off of the iMac, you will gain the benefit of having immediate access to the storage device to make whatever adjustments (power off, hard drive change, etc.) are necessary while sitting at your iMac.
Finally, unless you're already running on a wired basis from your iMac to your aTV, you might want to consider upgrading to do so. I think you'll find the benefits of wire much more compelling than wireless - especially when trying to move around content that comes in large containers like movies or uncompressed music files. Yes, the aTV currently only sports a 100 megabit ethernet port, but if you've got a reasonably new iMac it has a 1 gigabit ethernet port, as do the new Apple routers - I'd think this gap would be closed by the next aTV hardware release. Anyway, I would bet on the wired network over a wireless one any day for reliability, speed, connectivity, etc.
