This is said during the imposition (interesting word to use) of ashes when the cross is traced on one's forehead. I only occasionally get to hear this since one, or more, of the children is often sick on Ash Wednesday (today my daughter has a sore throat and swollen glands. I'm hoping it's not strep again). The ashes and the words remind us of our sinfulness and mortality.
Since Ash Wednesday usually occurs close to the end of seasonal depression time, it's always hard for me to get this into perspective (last year, Ash Wednesday was early and came in the midst of the depression season which made this even harder). My initial (depressed-type) reaction is "Of course, I'm worthless. Tell me something new!" I have to think more closely about it - that this is about not just a generic "being dust and worthless," but instead about having been created for communion with God and turning away from that to our solitary thoughts and desires.
Along with what I'm giving up, I'm also going to write every day during Lent. This isn't easy for me - words don't flow easily off of my fingers the way they do my husband's. So far, I've written for my weblog only when I've really felt like writing so it will be interesting to see what I can (or can't) come up with on the days when I don't feel like writing at all (like today).
It's easier to feel mournful and penitential when Ash Wednesday is early in the year and the weather is bad. Today, though, it's in the 70's (!) and beautiful. It's tempting to say, "Yeah, sure, I'm dust, but the daffodils are blooming!