I had experienced an issue with the DropBox client for Ubuntu 10.04 where PSD files over 1MB were not consistently syncing. Here is a solution which seems to have fixed the problem.
Keep in mind that this involves making changes to /etc/fstab, which if not done properly, could cause loss of data or other disasters. First, be sure to backup your /etc/fstab file, and read up on some fstab documentation.
In /etc/fstab find the root mount point which should have a forward slash as the 2nd field. Fields are separated by spaces or tabs on a line. Mine looks like this:
/dev/sda3 / ext3 errors=remount-ro 0 1
The 4th field is the options field. You will want to append the user_xattr option to these options. Options are separated by commas, with no space between them. So, in my case, it would look like this:
/dev/sda3 / ext3 errors=remount-ro,user_xattr 0 1
After saving this file, restart your computer and DropBox should work correctly.