The OU still has a significant number of students using IE7, and I have not heard any complaints about it being broken from them. Of course, that is using the OU's theme.
The only think I can think of is that IE has two rendering modes. I can't remember exactly what they are called, but something like "standards based" and "legacy". We made sure that the OU theme triggered "standards based" mode, in order to fix a lot of issues.
Sorry, that is pretty incoherent. Here is a better explanation: http://stackoverflow.com/questions/7155156/how-many-ie-rendering-modes-are-out-there. Basically, check which "document mode" your theme is triggering.