The InfoPath Team Blog has a great article on how to implement cascading dropdowns in InfoPath Forms Server (IPFS) without writing code: http://blogs.msdn.com/infopath/archive/2006/10/12/cascading-dropdowns-in-browser-forms.aspx. Unfortunately, this approach does not work in repeating context for the same reason Cascading Dropdowns are so hard in IPFS: there is no filtering on dropdown values.
However, if you absolutely have to have those in a repeating table or a section, there is a workaround. It is not pretty because it requires you to write code (hence the form needs admin deployment which is much harder), it pollutes your main DOM and it is less performant (postbacks are needed any time you change the "master" dropdown). However it works!
The approach relies on the fact that the data source for a dropdown can reside in the main DOM as well. Consider the following schema:
There are two dropdowns in the repeating table, one bound to myCategory and one to mySubCategory. If the values for the second dropdown come from a secondary data source, they will be the same for all rows which is obviously not desired. However, adding the source under myRepeatingTable solves the problem:myFields
Now all you need to do is to bind the second dropdown values to mySubCategoryValues and then write an OnChange handler for myCategory to modify those dynamically.
You can find a fully functional example here:
The zip file contains the ready-to-deploy XSN as well as the source code.