That's very interesting and something I'm inclined to do, too. Do you have any resources about that?
I have the same annoying problems - particularly as I like to offer up my bandwidth for Linux ISOs and creative commons works (I don't always contribute money to these projects (wife+kids usually see my wages before I do) so it's nice to contribute something even if the net effect is minute). Yet HTTP/HTTPS is completely unthrottled.
considering my ADSL only syncs at 6MB, and not 200MB or whatever, it's fine for my purposes. Most of my machines are also still connected with 802.11g, so locally is not really a big problem either.
I would think twice about using it as a straight fileserver though, because the ethernet and usb ports share bandwidth.
Of course I can't route my primary internet connection (about 330-340 Mbps typical throughput) over it, but Asus RT-N56U does that just fine.
Fileserver is not an issue, because there's about 300 Mbps total USB bandwidth and the NIC is just 100 Mbps. IRQs are a bit of an issue, full 100 Mbit bandwidth generates 2k IRQ/s, USB disk will probably take it to 8k IRQ/s, consuming a significant portion of CPU (maybe about 20%?).
But if 10 MBps (80-90 Mbps throughput) fileserver performance is enough, RPi is just fine. About 3.5 MBps over ssh, like when using for example sshfs. For comparison, 3.5 MBps more than enough to saturate "54 Mbps" 11g for example, which can do 24 Mbps under ideal conditions (~3 MBps).
I got sick of fighting my isp's throttling rules, which kicked in at the most frustrating and inopportune times, like halfway through downloading linux ISO's to play with on my raspberry pi.