Ha! I'm so glad i found this when i Googled "leopard dock crash". I had the exact same problem and trashing it from the the downloads folder restored the Dock immediately. Great.
I think i know where the problem may start. Leopard usually renders JPGs fine right? But when i edit one of my lil Canon pix in Lightroom, like changing color and metadata and then Save it in LR, the resulting JPG becomes a Leopard-crasher. If you even point to it in my Finder it crashes it, much less place it in the Downloads folder (which probably shows up in your's and my dock).