I have got an early Ethernet Shield v1.1 working today but as soon as I added a DHT11 Humidity Sensor I noticed some conflicts where the humidity sensor could not be read.
The Ethernet library here http://www.nuelectronics.com/estore/index.php?main_page=product_info&cPath=1&products_id=4
is what I am using.
I ran a simple sketch to read the DHT Sensor called DHTtester without the Ethernet library (Ethernet shield connected still) and it runs 100% with no errors or failures to read.
My DHT sensor is connected to Digital input PIN2 on the Arduino Duemilanove which I "assume" the Ethernet does not use?
My loop() reads the Humidity sensor and then checks Ethernet so both are not occurring at the same time.
I am actually only reading the humidity sensor once every 10 seconds.
Any help appreciated.