Haven't done much with getting Arduino online, but it's something I'd like to do for my next project. I was going to go pick up a built Ethernet Shield this week, but remembered a had an unused gateway router sitting in a box and wondered if I could scavenge enough parts to make my own shield. The router is a 2wire 2701HG-G and inside I've found the 4 Ethernet Ports are connected to a Delta LFE8731 LAN filter which then connects to a Marvell 88E6061 6-Port Fast Ethernet Switch and onto more chips and components than you can shake a soldering iron at.
So, any ideas or should I just give in and buy one ready made?
Most routers consist of a magic "System on a Chip" chip that has a 100+MHz ARM or MIPS cpu and several network interfaces all on one chip. Getting just an ethernet interface out of one would be next to impossible. And if you did, it wouldn't be compatible with the standard arduino ethernet shield anyway...
Now, in theory, if you have one of the routers with a built-in serial port (perhaps "not wired") you can re-flash your router with something like DD-WRT or OpenWRT, run a bit of customized software so that it behaves like a Lantronix serial-to-tcp converter, or similar.
I've been experimenting with this a bit myself, recently. (I haven't gotten far enough to say much, other than that I'm a bit frustrated by the state of the development environment for DDWRT.) I find it a bit mind-boggling that you can get a full wireless internet router with embedded CPU and such for less $$ than a wireless network module (and frequently for less than a wired ethernet interface), but I guess such are the properties of mass production.