You won't get a error from beta 7 for windows. It still allows you to load the iOS 5 ipsw. In order to get that iOS 5 seed you have to convert that .dmg to the ipsw using transmac which is a free download and has a 15 day trial but you don't actually need to register for that single usage.
This is how I prepped my device for ios5
Yeah, that's what I was saying, iTunes beta 7 for windows. As for the iOS 5 GM which I downloaded and found out that it came on .dmg instead of .ipsw I used powerISO to open the iOS 5 GM .dmg file and then drag out to my desktop the ios 5 GM .ipsw file which was inside the it.