There are quite a few resources out there to help with learning the Toolkit.  Whether you're a new user or a Toolkit Pro, there's something to learn for everyone here.  If I've missed anything, let me know:

• Joe Stagner's great How-Do-I videos (I count 13 Toolkit videos in the list): http://www.asp.net/learn/videos/default.aspx?tabid=63#howdoi
• Mohammed Azam has a good tutorial worth reading: http://aspalliance.com/830_Working_with_ATLAS_Control_Toolkit__Part_1
• The walkthroughs available from the Toolkit itself, see the menu in the lower left corner: http://atlas.asp.net/atlastoolkit/
• Marc has a video showing how to use the Toolkit in ASP.NET 2.0 Applications up on Channel9: http://channel9.msdn.com/Showpost.aspx?postid=199266

Enjoy!

UPDATE: Coding Atlas is another great blog.  The other takes the time to convert their experiences into walk throughs that are really cool.  Check it out!

UPDATE: AspAlliance also has a set of really cool videos on the Toolkit as well: http://aspalliance.com/videos/