Rumour is ripe on the interwebs, as is the norm, but in recent hope/fiction the mention of a successor to the DS has come up more than once. Some people predict we'll see some sort of announcement at E3, or at the next Tokyo Game Show.
All rumour aside, let's try to understand a couple of things. First of all the DSi has been out about as long as I've been blogging (not literally), so why would Nintendo waste the time and money on developing this upgraded version only to announce a new DS 2? Only a year into its life-cycle I'm pretty sure developers still have a lot they can achieve on the platform; especially given the recent leak of 3D gaming on DS.
Secondly, according to the NPD numbers for last month (Feb 2010) the DS sold over 613,000 units. That shows that people are still happy to keep buying it even though it's getting on in its years... just look at the PS2. Granted that Pokemon HeartGold/SoulSilver may have had something to do with the sales, but even so, people still love the DS.
I suppose we'll just have to wait and see what happens. Maybe the DS 2 will simply be just another upgrade, god knows Nintendo believes an extra few millimetres of screen size will be a huge expansive improvement, and why not throw another camera on there to, because clearly no-one owns a camera phone these days.