Recently had a comment asking for online resources for InfoPath 2007 Template Parts and only posted 2 links because they were easiest to find.  Since then I went looking a little harder and found the following and in my opinion the "Designing and using template parts" is probably the best one of the lot.  It links to about 6 or 7 online MS articles that cover about everything on template parts.

Cheers

Designing and using template parts
http://office.microsoft.com/en-us/infopath/CH101638151033.aspx

Lab 10: Creating and Inserting InfoPath 2007 Template Parts
http://msdn2.microsoft.com/en-us/library/bb267331.aspx

How to: Share InfoPath 2007 Template Parts and ActiveX Controls
http://msdn2.microsoft.com/en-us/library/bb409614.aspx

InfoPath Team Blog - Template Parts 101
http://blogs.msdn.com/infopath/archive/2007/03/14/template-parts-101.aspx

The SharePoint Factory - InfoPath 2007: Form Template Parts
http://spsfactory.blogspot.com/2006/07/infopath-2007-form-template-parts.html

InfoPath 2007: Creating a Template Part
http://blog.funknstyle.com/?p=503