Hi dkolbay, the latest version of the library (compatible with Arduino 1.0) is here (from the march 28th 2012 post up there ^).
With these kind of problems, it's always wise to take a step back in order to analyze the problem, then determine the simplest test possible, and then try to get it working.
Yours is obviously not working correctly or it wouldn't be spewing out zeroes. There can be problems somewhere in the communication, but the most obvious problem is that the clock signal (not serial clock) you are feeding to the ADS1211 is not correct. But let's first devise a simple test, as your sketch right now involves multiple write and read operations, too many opportunities for errors.
The simplest test would involve as few operations as possible and as simple feedback as possible. This could be 1) writing to the command register that Vbias must be on, making Vbias output some voltage and 2) reading this voltage with an analog Arduino pin. You can make a sketch to do this test and make it output a victorious message when the test succeeds.
If you want to feed a clock signal to the ADS1211, you'll have to configure one of Arduino's timers to do so. Timers are quite simple pieces of hardware, they simply count and when they reach a number, they do something. You can read all about it in the Atmega328's datasheet (it might seem overwhelming but keep in mind that timers are very simple devices, albeit with many configuration options). What you want is to 1) take a timer that controls a port that is not used by the library and 2) make it produce a square wave (thus output 0v and 5v with equal periods).
So if you have the test sketch that outputs a square wave to the ADS1211's clock input, writes to the command register and sets the Vbias high, and then reads the Vbias voltage on an analog Arduino pin, you can hook everything up and start testing. If it still doesn't work, you only have a few things to check. If it does work, hooray, and you can start expanding the code to fit your needs.
I dug in my sketches folders and found some code to output a square wave on a pin using a timer, I use it for producing tones with a speaker. The .h file (it's just a C file, you can open it with the Arduino IDE) contains the functions that set the timer and set tones. The .ino file houses an example sketch. It does a lot of direct register manipulation using bit math. The constants like "TCCR2A" that you see in the code are defined in the avr/io.h file (Arduino includes it automatically I believe), they are simply bits of code that write or read a register (in the "TCCR2A" case) or define where a port bit is in the register with a number between 0 and 7 (in the "OC2A" case), and _BV(n) is function that shifts a binary 1 n places to the left (1<<n).
I hope this helps you solve the problem, and if not, come back here with some more details.