While it hasn't been explicitly stated on any Grey's Anatomy episodes this fall, it is widely rumored that the sweater she's wearing belonged to Denny (one fan observes she said it smells like him), and represents a part of Izzie's life that is gone, but not forgotten.
It turns out this is true, according to one of the show's writers, Debora Cahn, who confirms it in the show's official blog. She also offers some anecdotes about the scenes in which Katherine Heigl (who plays Izzie) was forced to stand outside all day long.
In Seattle, this might not have been a big deal. But in Los Angeles, where most of Grey's Anatomy is filmed, it's considerably hotter. Below is Cahn's take on both the lovely Katherine and her struggling intern character:
"Izzie’s trying to get on with her life too, in an incredibly valiant way. But she can’t. It was heartbreaking, watching her standing outside that hospital all day. It was 95 degrees out when we shot it, and she’s wearing Denny’s sweater, so on top of the emotional devastation, we were a little worried Katie Heigl was going to pass out.
Katie was fine. But Izzie was wrecked. She thought that she could take back her life, through sheer force of will, but it’s too much. We all wanted her to walk through that door. We all wanted to believe that she could bounce back. Still be a doctor. Be a superhero. Step out of the wreckage, brush herself off, and walk on. But she can’t."
At least not yet. Maybe after her empowering visit with Denny Duquette, Sr., during last week's "What I Am," she'll finally summon the courage needed to walk through those doors.