Currently fulfilling this role is my web/dev server massively overpowered, massively massive.
Wouldn't it be nice to run my Printer from a RasPi instead, 1W at full load? Sounds a little cheaper to run than 800W at full load.
On the file server front I fear that the throughput on the USB to Ethernet might not be sufficient for quick large transfers, but will be more than enough for basic file serving from a USB disk.
Hurry up and release the RasPi, my electricity bill for the current server alone will pay for the cost of more than one.