Preamble: I'm not sure that I understand exactly what you are looking for here but I'll share my findings anyways.
I was able to retrieve the iframe URL for the reviews and was able to see the reviews iframe after embedding it into an .html page. I used the following attributes to retrieve the iframe URL:
Operation=ItemLookup&
ItemId=1451648537&
ResponseGroup=Reviews&
TruncateReviewsAt="256"&
IncludeReviewsSummary="False"&
Version=2011-08-01 <= important: can't be less than this version
AssociateTag=<YourAssociateTag> <= required when using this version, can be anything (not verified by Amazon)
The relevant part of the response:
<Item>
<ASIN>1451648537</ASIN>
<CustomerReviews>
<IFrameURL>http://www.amazon.com/reviews/iframe?akid=<YourAmazonKey>&alinkCode=xm2&asin=1451648537&atag=<YourAssociateTag>&exp=2012-01-06T02%3A10%3A38Z&summary=0&truncate=256&v=2&sig=kjWPue1N75%2FiI1hW67XYApWxnKeT2tlT%2FJ1rw4WLlUo%3D</IFrameURL>
<HasReviews>true</HasReviews>
</CustomerReviews>
</Item>
Note that you will not be able to use this iframe URL due to the fact that (1) I've removed my Amazon Key and the Associate Tag that was used to create the signature and (2) it expires 24 hours after the call.
If you use the same attributes that I've used, paying close attention to the Version and AssociateTag fields, you will get a result with an iframe URL included.
I then embedded the url in an iframe:
<html>
<body>
<iframe src="http://www.amazon.com/reviews/iframe?akid=<YourAmazonKey>&alinkCode=xm2&asin=1451648537&atag=<YourAssociateTag>&exp=2012-01-06
answered 2012-01-05T02:55:27.423