Let's look at Japan + US.
Japan
Total
Wii - 2,476,552
360 - 362,383
April
Wii 357,514
360 11,574
US
Total
Wii - 2,470,000
360 - 5,401,000
April
Wii 360k
360 174k
Japan + USA totals
Wii - 4,946,522
360 - 5,763,383
360 is 816,861 ahead as of April US and most of May JP for the two territories.
(obviously we have Japan data to May but US data only to April)
April totals
USA + Japan
Wii 715,514
360 185,574
April Wii gain (US+JP)
529,940
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Assuming that the Wii will outsell the 360 by the same amount in May and June in both the US and Japan as it did in April, which would equal a gain of 901,940 to the current totals by the end of June.
Basically, Wii will surpass 360 in USA+Japan by the end of June.
Unless the 360 can hold off the Wii LTD in Europe by the end of June (who knows?), the Wii will be ahead worldwide by then.