CodeIgniter 2.x sessions and Internet Explorer

internet-explorer codeigniter session

A few things:

CodeIgniter has no issue with sessions and IE. IE has issues with an underscore in the cookie; it has nothing to do with CI (version 1.7 or 2.x).
IE has security features that prevent cross-site cookies. So you need to ensure these are no kicking in and destroying the cookie before it even starts. Edit: see here for more details: http://codeigniter.com/forums/viewthread/121637/.
Get your dealer to test the site via Firefox. Are you sure it is actually an issue with IE, or just an issue in general?
Set sess_match_ip and sess_match_useragent to FALSE, as that could cause the issue you have described.
Make sure you have set cookie_domain, cookie_path.
Make sure you have set cookie_secure to FALSE (unless the <iframe> is going over https).

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