I just finished adding retry capability to the DHCP library.  It will, by default, resend DHCP requests every 2 seconds, and timeout after 60 seconds.  I've also removed the need to declare a Dhcp variable. Take a look at the included example to see how to use the new library.

       Download here: Arduino DHCP Library v0.2

I hope this release solves the outstanding issues I know about.  I'm also looking for feedback on additional enhancements - ARP conflict detection, lease renewal, etc.  If you know of something that doesn't work or you would like to see added, please let me know.

Enjoy!