People seem to look at US sales and assume the same can be extrapolated to the rest of the world. Basically the only place where the DS is still doing decent numbers is North America. At least so far Nintendo doesn't seem to be nearly as successful in developing countries.
Here is the regional breakdown for the first 6 months of Nintendo's FY (April-September).
DS
Japan: 10k
Americas: 790k
Others: 190k
Total: 980k (Yes it adds up to 990, but that is likely due to rounding) compared to 1142k sold according to VGC
So about 80% of the shipments for the first half of their fiscal year went to the Americas. And that percentage has generally been increasing over the last year and a half at least.
The last quarter that we got PS2 figures on their own was Jan-Mar of this year. For that quarter it shipped 600k compared to 460k for the DS. We also know that Nintendo's DS forecast for this FY is 2.5 million while the PS2 one is 3 million. So far the DS is down 62% in the first 6 months of the fiscal year. This would suggest a 1.94 million FY if the drop is consistent for the year. I expect it will probably excede that, but not by the kind of margins it needs to make substantial gains on the PS2.
Sony has shipped 6.3 million PS2+PS3's in the first six months of their fiscal year. That corresponds to 3335k PS3 sales according to VGC. Even if we assume 5 million of that number is shipped PS3's, that would still have the PS2 outshipping the DS by over 300k for the half year.
None of the figures that are available suggest the DS has outsold the PS2 or that it is even outselling it quarter to quarter. Though it should take this quarter quite handily as the PS2 hasn't experienced substantial holiday boosts in quite a while, and with DS sales so concentrated in NA it will likely have an incredibly large increase.