In the past, Nintendo has adhered to a pretty predictable cycle on redesigning their handhelds. Every year and a half to two years or so, it's time for a new model. But no handheld has ever been quite like the DS. It's everywhere. Your grandmother just may own one (or more), and because of that incredible success, Nintendo can afford to break out of that cycle and take a little extra time. Sales of the DS in Japan may be flagging a little of late, due to the PSP actually selling, but the system still boasts extremely robust numbers and is consistently in the top three there amongst hardware. The story is the same here in the U.S.: the DS continues to sell like crazy. Why put out a new DS now? It seems to make much more sense to simply wait a while, until sales actually start to drop off.
In the meantime, Nintendo boosts sales of the Lite by releasing new colors, which they have suddenly started doing a lot more outside Japan. That seems to be another sign that we're still not quite ready for a redesign. Nintendo may make some less-than-brilliant decisions at times, but announcing a new design mere months (or weeks, if we see more colors this year, as is rumored) after the release of new colors may have a negative impact on sales. Sure, that new sparkly-striped-gold-plated rainbow Lite is spiffy, but if you're about to buy the new Ultra Lite anyway, would you drop an additional hundred-plus dollars? Probably not. You'd wait to check out the new model instead.
Some could argue that colors are now going to multiple regions in order to get rid of back stock, but c'mon -- are there really piles of DS Lites lying around unsold? Considering that we still have occasional DS shortages, it doesn't seem likely. Also, we're still getting brand new colors on top of the shades that are just now becoming available in new regions, and it's not like those were piled up. Would we really get new colors now if we were about to see a new DS? It just doesn't make sense.
Next year, maybe, Nintendo will get around to tweaking the DS design further. This year, we'll see more peripherals and accessories instead, and a focus on the Wii, because there's just no reason to do anything else.