If you run across this error you will find it in the event log when you try to execute a call to the PowerPoint Automation Service Application for a conversion request. The error occurs because the application pool account that the service is running under doesn't have the needed permissions (ie, they weren't setup by SharePoint properly in Beta) to create directories here:
If you look at the permissions on the directory, you see that Machine Administrators can do anything they like on this directory. Bummer. You now have to add the service account (or change the application pool to some other account that you already have given this access…like the Farm Account) for the PowerPoint Automation Service Application into the local machine administrators group. That seems a bit much don't you think?
Hopefully it will be fixed in RTM,