if nintendo can supply enough for everyone who wants one at christmas, i'd say mid to late 2008, if not, it will catch up a lot next christmas and be passing in early 2009.
if Wii demand falls however, it could be late 2009-10.
currently i'm more interested in the 'other' markets (well i do live in the UK).
ignoring all the things that change sales figures, Wii is on track to pass 360 in 'others' as a whole about 16 weeks from now, which will be shortly before the christmas rush, so if Nintendo have enough Wiis it will then soar ahead...if they don't 360 might pass it back (though not by much i'd have thought)
again, with enough supply it should surpass 360 in Italy this year (current numbers show about 18 weeks, but that's mid November so the weekly sales will be very different by then)
In the UK, Scandanavia, other Europe, Australia and 'others' the Wii has little chance of surpassing 360 till mid 2008 at the earliest, mainly due to 360s early popularity in UK, Aus and Scandanavia. but also the other regions general PS brand popularity (or possibly just there are no Wiis)











