SD cards and their embedded disk controllers vary wildly. They can do weird stuff with sudden loss of power.
I've had brands with the controller choking after 2 days. I've had cards run for a year no issue.
Flashybrid. Go to the Developer forum here, we have a huge thread where I show how to install and setup flashybrid.
How does it do It?
It Runs the SD card READ ONLY. At bootup we create a ram disk, and copy any files that need read/write access (SQL Database for one). Bind mount points redirect access to files to the ram disk. Anything else can be accessed read only. When you cleanly shutdown, the ram disk is synced back to the SD card. You can invoke the 'fh-sync' command to commit the ramdisk back to SD anytime. Two additional scripts allow you to remount read/write at any time, and back again to readonly mode (mountrw and mountro are the script commands in search path already). Apt-get install flashybrid, then configure it

It makes it rock solid. Only changes you decide to commit back to SD card stay, otherwise is a failure scenario like power loss, it boots up in last good state. This is just like a Cisco router, you flash the config back when you want it to be, otherwise 'test' your changes.
Also, with MySQL running from a RAM disk, the call processing speeds get even faster

Keep the Sheeva on a UPS, and setup flashybrid, so far it seems bullet proof.
Everyone has my stock image, I figured somebody out there would install my base image, add the flashybrid as I've documented here and then I can post that image as newer. I just can't do everything, so hopefully someone can pickup the slack here

I want flashbyrid to be a standard part of PlugPBX. I put it into place on my own systems, works well. I had a batch of SD cards that were just junk, they'd die after 2 days, but with flashbyrbid, my systems are up almost a year now. When I'm inspired to finish the install/setup shell script, I'll add flashybrid installation and configuration as part of it (its one of the last pieces remaining).
I looked at using JFFS (does not scale to large filesystems), I've looked at other solutions, but flashybrid offers the best compromises - because you can still hack the system, add/remote programs. The only consideration is that you 'add' any files that need read/write privileges for the system to work, and watch the ram disk sizing and usage. Its pretty easy to tune.