I've noticed this problem with AT&T wireless's website. When I first load up the website (attwireless.com), the textbox for "Wireless Number or Member ID:" is highlighted. While that textbox is highlighted, I am unable to use the shortcut key to log in (command + \). If I tab out of that text box (ie. to go to the password box) or if I click somewhere else on the page, then I can use the shortcut key to log in again. I've noticed this with Safari 4 and now 5.