Hi Steve,
In IIS 7.5, double click the Authentication icon, then right click the Anonymous Authentication line and choose edit. What do you see there? Is it set to use IUser or ApplicationPoolIdentity?
If it is set to use application pool identity then the identity user on your application pool is the user who needs file system permissions and who will own newly uploaded files because that is the user that the web process executes as.
A lot of this is changes in IIS 7.5, in IIS 6 and 7 the default was to use NETWORK SERVICE as the identiity on the appication pool and then the file permissions needed to be for NETWORK SERVICE.
If all else fails you can configure your app pool to use NETWORK SERVICE and then set file and folder permissions for NETWORK SERVICE rather than IUser.
Hope it helps,
Joe