I understand that Metro limits the access an app has to the user's computer. Can anybody verify this, as I am considering creating an email client that will work with at least yahoo/at&t.net and gmail. Will I be able to access a user's email (assuming they provide their username and password, of course) with a Metro style app, or will I need to stick with creating an "old-fashioned" Desktop app for this scenario?
Metro can easily be used to create email clients. For something like this, you would be syncing data between the mail-server and the client.
Metro apps have limited access to the filesystem. But they can access Application Data Storage and isolatedStorage which can be used to store data pertaining to the application offline. This helper library helps store structured data in isolated storage, and might suit your needs.
For your scenario, a metro application would be a very good fit. However, the reach of metro applications will be decided by how well Windows 8 does after launch.