Hello !
The new DHCP class for Wiz5100 is a great improvement ! However, due to the limitations of the Wiz5100 chipset (?) this is not 100% reliable, because the dhcp process should be run only when a network cable is (re)connected. For an example, the Fez board is well connected with the user’s router, and for some reasons the router is power cycled, and when it’s backup the dhcp lease is not good anymore…
I am pretty sure from the Wiznet datasheet that checking the cable status is not possible through software (register). Is it correct ?
If it’s not possible using software, I will work on some hardware trick. Maybe loop the “link” status led to an input pin and monitor the result, to reset the Wiz5100 stack and run the DHCP once the network is up (and re-run it everytime the network goes down for more than 30s and back up, as the standards usualy do). Has someone some working code already 
?
