Why is App_Offline failing to work as soon as you it starts loading dlls?

Could anyone please help me with this. On the production site app_offline.htm works only till you start uploading dlls. As soon as you start uploading dlls it throws following error"Could not load file or assembly 'SubSonic' or one of its dependencies. The process cannot access the file because it is being used by another process. Now this clearly shows that IIS is still trying to serve the aspx page?? Is there anything I am missing here?? Any help would be appreciated. I have spent hours googling but to no avail. Thanks in advance. Manisha