There is a bug on dhcp3-client that describes this same problem. Though the last comment in that bug claims the problem is still unfixed, I believe it actually is fixed in dhcp3-client but unfixed in isc-dhcp-client.
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=508804 is the bug against dhcp3-client. The same patches suggested in that bug would fix this one. -ben
