Update 7/3/2011: This library no longer works with current builds of Arduino. At some point in the future I *may* decide to create a new revision, but my current focus is not on Arduino development. I believe there are one or more DHCP stacks based on my initial work. If you need DHCP support for your Arduino project, I would recommend you search for one of those libraries.
I made a small update to the Arduino DHCP library to calculate the “seconds elapsed” field in the DHCP packet based on when the Arduino starts sending DHCP packets. This may fix some issues with using a Mac as a DHCP server. Special thanks to Arduino forum member smartperson for the suggestion.
Download here: Arduino DHCP Library v0.4
Previous Versions: v0.3, v0.2, v0.1
Additional instructions can be found on the original DHCP library post.