Is it possible to set field properties at Role Level?

 We would like to make some fields on the Account read only for the Sales Rep Role. But when we do that via the Admin link, the field becomes read only for all Roles including Administrator.

Wondering if there is another way to set these properties so that only the Sales Rep role can't modify certain fields on the Account tab and others can? Appreciate any responses or pointers.

Thanks

-Sumeit