I am starting to see this error frequently also. Here is the message in .xsession-errors:
The program 'gnome-settings-daemon' received an X Window System error. This probably reflects a bug in the program. The error was 'BadMatch (invalid parameter attributes)'. (Details: serial 3799 error_code 8 request_code 15 minor_code 0) (Note to programmers: normally, X errors are reported asynchronously; that is, you will receive the error a while after causing it. To debug your program, run it with the --sync command line option to change this behavior. You can then get a meaningful backtrace from your debugger if you break on the gdk_x_error() function.) I run an up to date Squeeze.
