If you have experience with HTML or XML or similarly based languages, it's not exceptionally difficult, especially if you have web design experience. If you're coming strictly from a WinForms background, the learning curve is a bit steeper. I am currently learning WPF myself and since I come from an extensive web development background, I'm not finding the concepts all that difficult to master for most of them. I've found the online tutorial videos at WindowsClient.net to be exceptionally helpful as well, if a bit poorly organized.
