We have experienced the same problem. It seems that this doesn't occur on Designer 8.0/Acrobat 8 but with Designer that comes with Acrobat 9 (Designer ES?), this happens every time you add script to the calculate event of the subform. I believe that when you add a script to the calculation event, the Designer adds access="readOnly" to the subform itself, so all its children becomes disabled. Once again, on Acrobat 8 this doesn't effect at all but on Acrobat and Reader 9 all childrens of the subforms are unaccessible.
If you encounter thsi problem, check the XML source of the subform. If it has access="readOnly", remove it.
<subform layout="lr-tb" minH="17mm" name="someSubform" w="170mm" access="readOnly">
In my opinion, this seems like a bug and should be fixed on next release.
EDIT: I think this problem occurs only, if the form is saved as dynamic so static forms doesn't have the same effect.