Some of the system I have encountered problems with Ubuntu static IP addresses have had multiple NICs and sometimes I've been referring to the wrong NIC. But I have had problems with Network Manager seemingly not remembering, or not applying manual NIC changes. The automatic Network Manager seems to have overridden my manual configuration settings.

Ubuntu Static IP Content has been written to help with Network Manager not working reliably in Ubuntu 8.10 and 9.04.

I'm giving up on Network Manager, 9.04 has lots of problems with Network Manager. Try wicd here.
 
apt-get install wicd
reboot
 
Well; I've had similar problems with wicd. Network Manager doesn't work reliably in 8.10 and 9.04 using:

/etc/network/interfaces like PC Bubbles and PC BubU.
/etc/init.d/network restart