Unfortunately, with modern distros like Ubuntu, this is made very much more complicated thanks to the interactions of HAL, udev and friends.
I don't have a ubuntu desktop machine in front of me right now to test, but (theorising) it's possible that you can simply override how HAL decides to mount a filesystem by adding a line to your /etc/fstab -
First find out the id of your mbed using 'blkid'
Having that, the line may look something like:
UUID=d096014a-182a-4c4c-bf71-0da773c8d876 /media/mymbed auto sync 0 0
You'll want to ensure you have created /media/mymbed
(Completely untested, and probably will not work!)
Update: Turns out this was in the FAQ's all along! http://mbed.org/support/FAQ0006
Quick question. I want to add a line to my fstab so that new file writes to the mbed get synced more quickly. Does anyone know what the line should look like that I should add to fstab?
I did find a post from Mike Sheldon that hints at the possibility. Below is the post that I found.
Thanks for any helpful tips,
Jim