If you're like me, you probably have several applications open at once. For example, I often have Excel, TextWrangler, Photoshop CS3, Safari, Firefox, Tiger Woods PGA Tour '08, and iTunes open at the same time - and that's not including Entourage, Adium, Twitterific, and other apps that are open all the time.
JoeSoft's Jax iTunes plug-in caused iTunes to crash when I installed Leopard. JoeSoft's Web site says they "are in the process of updating Jax to be Leopard compatible. This update will be posted shortly after Leopard's release."