I have been writing a lot about XamDataGrid on my other blog recently. My most recent post shows how to save and load the widths of columns/fields across runs of the application. This is just a temporary solution that will eventually be replaced by built-in functionality in the control, but until then, at least we now have a way to do this with ease. My implementation even supports saving/loading the width of fields when showing hierarchical data (i.e. multiple related tables in the same grid). Enjoy!
Persisting field widths in XamDataGrid