In my opinion it would have made a great portable 3g wifi router/ap with fun features but no more of course :-)
If Ethernet wasn't USB and there was supported onboard WiFi, or any onboard Wifi, maybe. 3g support wouldn't be a whole lot better as the only way to connect it would be via USB and many cell based data USB adapters pull a lot of power from USB (some have batteries to overcome their peak power draw.)
The main issue with USB for NICs is that they can be very
flaky when used in such a manner. Routers are generally expected to be always on with no drop-outs. A USB adapter often doesn't really offer that, especially once you start putting multiple of them on a USB bus. The USB offerings on a Pi only has 2 ports and should be expected to offer no more than 100ma per port. People using the Pi are encouraged to use powered USB hubs with the Pi. So, to support USB WiFi and a 3g adapter, you're going to also need to add a powered USB hub, which is also another layer of possible failure. Also, all data is going over USB at that point. With USB, 400Mb doesn't simply divide out to 1/8 for MB, you'll generally get a max of 33MB/s, shared, across everything. So, if you have a 3g adapter, WiFi and Ethernet, I think you'll get a max of 15MB total throughput combined, not each way, or 7MB symetrical, actually less due to contention losses. While that sounds like plenty, that's also counting anyone using wireless accessing something on wired, locally.
So, you'd have a very slow router with WiFi that still needs some other kind of power for a powered USB hub.
I would think that you'd be better off with something in the Atom range for that, something with PCI-E (mini or full sized) for your WiFi connection. Maybe a very small netbook would work, it's not quite as small, but has built in battery backup! Or an Alix board, some draw as little as 5w.
I'm sure the Pi is cheaper, but (aside from the lack of Arm support that's reported to be in the works) I wouldn't expect it to be a great router.
(edit: fixing Mb Vs. MB on a couple labels.)