I have the same problem...
What I have found out this is a major problem due to Iphone use MobileSafari and everywhere I have been searching, at the Joomla-forum and elsewhere, this is something with the js-file.
I found this on another page but I'm not a expert in this type of codes... http://www.parorrey.com/blog/php-development/resolve-mobile-safari-greyed-out-file-field-issue-in-iphone-ipod-ipad-by-using-picup-app-to-upload-images/
Could this be the solution? Is there anyone that has tried this?