There are various modules on the market for connecting devices to ethernet LANs.
Some are Arduino-specific, i.e. the shield from Sparkfun, DEV-09026, $46.
Others seem to be "generic", and connect (on the device-to-connect-to-LAN side) via a serial interface. (Are there other options?). The Sparkfun /WIZnet Serial-to-Ethernet Gateway looks distinctly cool, especially at $30, complete with built in webserver.
Pros and cons? I always prefer a simple, general answer. I use ModernDevice RBBBs, anyway, so a shield's easy "plug in", the easy management of the physical problems of connection are not going to help me, anyway, are they?
Are there other advantages to a shield that I'm missing?
What should I look for in an Arduino to LAN module?
I don't mind having to configure the module "the hard way", i.e. by connecting it up to a big PC and setting a fixed IP address, etc.
The arduino ethernet shield has libraries made for it. You can get it working in a matter of minutes.
Google for 'arduino ' and whatever device you're looking at to see how well supported it is. You may find a little more money will make it loads easier to use. You might not though. Have a google.
I'll second tep's suggestion. If you look at ladyada's Ehternet shield, you'll find it's actually using the Wiznet module. And it's fully compatible with the standard Ethernet libraries. Which can be a big plus compared to, say, the ENC28J60 that some people are using because it's cheaper. The Wiznet doesn't need many connections to the Arduino, so it's not a big hassle to wire one to something like the RBBB.
Based on my experience, I recommend that you wire the modules so the Wiznet reset is done under software control, using a digital pin. It's a bit of a bummer to "waste" a pin for that, but it enables you to create systems that don't have to have the reset button pushed to recover from networking problems.
Another plus, compared to the serial-to-Ethernet, is that you can build custom webservers and clients that interact directly with the Arduino hardware, instead of going through serial port messages. Or even both: I have a system running now that gathers weather data at a remote site, and posts it to an SQL database on a PHP-based server hundreds of miles away. At the same time, it's running its own small webserver, so I can surf in and check on how it's running without needing to drive out into the desert. I could do the "monitoring" part via a serial-to-Ethernet, but I doubt I could do the client: I'd probably need to create a PC app to communicate via the virtual serial port and act as the web client.
Wiznet has a U.S. mail-order site where you may be able to find a better deal on the modules. They happened to have a nice sale on the WIZ811 modules when I bouoght them last year, and I got 3 modules for about the price of two from most other vendors. Plus they carry stuff that few (maybe no) vendors stock.