Finally I've solved. I'm still using UIPEthernet sincee my problem was the use of DHT library (last rev) with UIPEhernet lib.
I was having a restart of teh ETH port each time I was reading Temp/hum. I've changed into SimpleDHT lib and now it's working fine. I'm using a mix of W5100 and ENC28J60.
If anyone is interested in trying out the implementation using UDP packets sent to the subnet broadcast address, check out my pull request https://github.com/mysensors/Arduino/pull/66
