I have never had to verify tab order using Selenium.
Using modifier key like the SHIFT key requires Selenium to be using native events. I was not able to get this test to pass on FF using 2.21, but newer versions of Selenium should work. It does work in IE8.
Send Shift Tab To Current Element is begging to be implemented in Python, but I wanted to give you something easy to try out.
Good Luck,
Kevin
*** Settings ***
Library Selenium2Library
*** Test Cases ***
Shift Tab
Wait Until Page Contains Element id=Passwd
Click Element id=Passwd
Focused Element Id Should Be Passwd
Send Shift Tab To Current Element
Focused Element Id Should Be Email
*** Keywords ***
Focused Element Id Should Be
[Arguments] ${expected id}
Should Be Equal ${focused id} ${expected id}
Send Shift Tab To Current Element
${keys}= Evaluate selenium.webdriver.common.keys.Keys selenium
${s2l}= Get Library Instance Selenium2Library
${actionchain module}= Evaluate selenium.webdriver.common.action_chains selenium
${action chain}= Call Method ${actionchain module} ActionChains ${s2l._current_browser()}
Call Method ${action chain} key_down ${keys.SHIFT}
Call Method ${action chain} send_keys ${keys.TAB}
Call Method ${action chain} key_up ${keys.SHIFT}
Call Method ${action chain} perform