First of all lets look at some facts.
*The DS and PSP have combined sold 64.1 million units world wide compared to 20.5 million for the three consoles.
*The PSP alone has sold more that the big three combined
*The DS is the fastest selling system of all time essentially. It's breaking records across Japan and Europe. It shows no signs of slowing down.
Now of course the issues that can't be ignored is the head the start the handhelds had over the consoles. So the consoles will catch up eventually right. Well lets just take a look at the last month in the US and the last week in Japan to see if they are in the process yet of catching up. The first numbers are combined DS+PSP sales and second are combined Wii, 360 and PS3 sales.
NPD April Sales in the US: 654k to 616k
Famitsu Harware sales Japan May 14-20: 138k to 68k
So the consoles, minus the PS2, aren't catching up at the moment. The lead is actually increasing for the handhelds assuming you'd find a similar situation in Europe, which is a safe bet. But console generations are marathons. The PS3 and 360 will certainly do better down the road as the prices come down and the big franchises come.
The big reason I think the DS and PSP will combined outsell the three consoles comes down to them being the systems best position to break out in growing markets like China and India. You don't need TV's to play them, and they're cheaper than the consoles and will always be so. The DS in particular I can see breaking out in areas that aren't concidered major video game markets. It's also worth mentioning that consoles are a houshold item while handhelds are an individual's items, so there will be households with more than one of the same handheld system, expecially with regards to the DS in households with multiple kids. It's a new age of gaming where handhelds are going to take a bigger chunk of the market. Consoles will still sell more software, but for hardware I think handhelds take the lead.