What browsers Google support can be very useful for client projects. Already having started phasing out support for IE6 they are only supporting IE8 and above for Google Plus accounts (presumably as XP use is still fairly high).
From their support page:
Supported browsers
Google profiles are supported in the following browsers:
Windows: Chrome, Firefox 3.6, Internet Explorer 8, Internet Explorer 9
Linux : Chrome, Firefox 3.6
Mac: Chrome, Firefox 3.6, Safari 4
This also means any Google +1 buttons on your site will not show up in IE6 and 7.