I'm running 10.04 LTS and the ubuntu-sync process is really using a lot memory and cpu resources, especially as time goes on during the day.
When I start out the day, (and do a top) the CPU% is typically 6.0 and %MEM 0.8, but as the day goes these numbers increase until my notebook is all but unusable, at which point I have to either re-boot or kill the process. Just today it was using 1.9G of memory. Using System Monitor right now, and looking at memory usage the MiB number for ubuntu-syncdaemon is only going up.
Same problem on Ubuntu 11.04. UbuntuOne get 25% of my 4GB of memory to synchronize. I'm using an HP EliteBook 8640p basic configuration. I don't have the same problem on my Dell XPS1330 with the same Ubuntu version.