Hi John,
I have the same setup as you do, I have a WRT54G running Tomato 1.28. I have two of the obi110 in two different locations and network and the other is connected to a switch and form my switch to my Cisco 1700 series router. I noticed that both tend to reboot at least once a day. Longest they seem to run without rebooting was 17hrs. It wasn't until I manually changed the configuration settings that it stopped rebooting.
Here's what you do, go to:
System Management > Auto Provisioning > ITSP Provisioning > Method >
Uncheck "Default" and set Method to "Disabled" from the drop down menu.
Do the same things for the following settings below.
System Management > Auto Provisioning > OBiTalk Provisioning > Method >
What this does is sets the obi to be configured manually, not through the obi website. I had changed these settings to solve a different problem but it seems to solve both problems, the rebooting.
It's a good idea to backup your configuration settings before you do any changes and keep them in a safe place. Never hurts to backup.