Nibbles are a series of "snack tutorials for hungry designers" that cover the use of Expression Blend to build WPF and Silverlight content.
Celso is an amazing interactive designer working at Microsoft who is responsible for the beautiful sample applications that ship with Expression Blend and did some of the earliest design explorations for Silverlight. He also delivered two designer sessions at Mix07. The first session "DES02 - Building Rich Web Experience with Silverlight and Microsoft Expression Studio for Designers" (Play (Silverlight) - Launch PPTX) and discusses the ins and outs of how to create stunning looking sites and Web experiences with Silverlight and Microsoft Expression Studio. The second session "DES07 - Design Rich Client Experiences with Expression Blend and WPF" (Play (Silverlight) - Launch PPTX) covers the use of Expression Blend to create interfaces using media, controls and rich layout.
For those of you who attended the Belux MSDN event last week on Expression Blend & Silverlight, the tutorials are an excellent starting point to familiarize with Silverlight and Expression Blend.