If you ever receive the error "Multiple section types are not supported" from either InfoPath Design Checker or when you try to upload your template to MOSS then you will probably find that you get very cross with InfoPath, at least I did!!

 

Anyway, firstly it seemed to be that InfoPath suddenly decided that Forms Services no longer supported having repeating elements within repeating elements (repeating tables/sections).
There are 2 solutions as follows :
1. Restart InfoPath Designer and try again (sometimes this works)
2. Recreate your entire parent & child repeating section/table

 

hope this helps people avoid the pain that I have just had for the last 2 hours!