IIRC - XP pre SP2 is unsupported.
XP SP2 and XP SP3 are under Microsofts "Extended" support for another few years (until 2012?). This basically means that you can still raise a Microsoft support call (Premier customers etc) for XP, and by association IE6.
However, MS has published IE8 as a Critical Update on Windows Update and you may struggle to get a fix from Microsoft for any problem you may encounter. If it turns out to be a bug in IE6 then MS will not be fixing it (security fixes only under Extended support).
Start working on your IE8 rollout plans. It's stable, renders better than IE7 and is toe-to-toe with FireFox (without the plugins)