summary:
I think it is the /data/local.prop.bak file that grows and grows and eventually fills up the /data partition that causes this problem. Rebooting the phone or doing a 'rm /data/local.prop.bak' is a workaround, but not a solution. It is possible that de-garbaging your /data/local.prop file will solve the /data/local.prop.bak problem. I am seeing if that solves the problem on my phone.
details:
I have a Motorola Flipside and I have been getting the "Device storage full" messages about once a day as well. Rebooting the phone seems to make the problem go away for a while.
Using a terminal emulator, I did a df commands, which showed that my /data partition has lots of free space right after bootup, but has very-little free space once I get the "Device storage full" message. All other partitions seemed fine and RAM usage seemed fine. The problem is the /data partition only.
Then, to diagnose what is taking up all the space on the /data partition, I used du commands, which showed that the only significant growth in storage used was for the /data/local.prop.bak file.
I imagined there's a bug such that the phone improperly backs up /data/local.prop in a way that /data/local.prop.bak indefinitely grows.
I looked at my /data/local.prop file, which seems do have some "ro.sys.*=0" lines, and then I have a lot of binary (non-ASCII) garbage and "=0" thrown in there. My local.prop is about 16.8MB. From my understanding, prop files are only supposed to contain ASCII name=value pairs. I have de-garbaged my /data/local.prop file and I will see how that goes.
edit1:
By the way, my de-garbaged /data/local.prop file is now only 288 bytes.
edit2:
Looks like /data/local.prop.bak is still growing (maybe slower than before), and as far as I can tell there is just repeated garbage in there. Looks like /data/local.prop.bak is completely useless. I could try to investigate what app or android process is buggily appending to /data/local.prop.bak, but I'm tempted to do a "chmod a-w /data/local.prop.bak; chown 0:0 /data/local.prop.bak" as a quick-fix to prevent any modification of the file.
edit3:
In order to mess with /data/local.prop.bak like I have (and also to do storage diagnosis with the 'df' and 'du' commands), you phone will need to be rooted and will probably need busybox (which usually accompanies rooting your phone).
edit4:
It's been about a week and making /data/local.prop.bak a zero-size unwritable file has prevented /data/local.prop.bak from growing. I have noticed no ill effects in the operation of my phone. I do wonder that process/app garbaged up my /data/local.prop and what process/app was making those worthless backups.