iFrames and frames in general are not search engine friendly and I would not expect them to be indexed. However, that doesn't mean they won't be indexed now or in the future. If your goal is to keep that content out of the search engines you should be explicitly blocking it with robots.txt or the x-robots.tag header.
FYI, blocking this is silly because:
1) Content has nothing to do with PR at all
2) If the form covers content related to the site you're removing keywords and content from your website that can help you rank better.
3) Frames are not very accessible so you're alienating some of your users.
My answer is still don't block them. You gain nothing and only potentially hurt yourself by blocking pages that have to them and from them. Internal linking is important and clearly being overlooked here.
But if you're going to block them using the x-robots-tag header is ideal as robots.txt is not always honored as you would expect with the search engines. Of course, using both is probably the safest bet.