Many people report that Ubuntu Server suddenly reverts its static IP address to dynamic (via DHCP). Here are some discussions:
static ip reverts back to DHCP (ubuntuforums.org).
Static IP reverting to DHCP on Ubuntu Server 8.04 (www.howtogeek.com).
This happened to me today with Ubuntu Server 9.04. Rebooting the server after installation have solved this problem.
