As long as you understand the security concerns involved with such a login, not to mention that some folks may be hesitant to transmit their data over anything other than a https connection.
Depending on how you want to implement it, the template path is:
skin\common_files\customer\right_bar.tpl
skin\common_files\customer\authbox.tpl
Just another FYI - spend some time with webmaster mode and you will be able to determine which template does what.
HTH