Nintendo's launched a website for Mario & Luigi: Partners in Time on DS. Heck, it's probably been up for a little bit, but we only just spotted it, and we're mentioning it because it has quite a bit of gameplay footage, and because it's made up like a time machine with levers to pull and buttons.
Go there and you can see, for example, how Luigi and Mario end up going back in time (in short: Luigi's a bit of a spack), observe the battle system in action, meet the bad guys who have kidnapped the Princess this time, and witness a few of the different moves you can perform when you combine Mario and Luigi with their younger selves - in a story that would have Sam Beckett rolling in his grave if he wasn't already flying around on spaceships fighting early Klingons and the like.
Sadly the game isn't out here for a little while yet, but Nintendo has pegged a date for it: January 27th 2006. Time travel won't be necessary to get it sooner though if you really want - it's out in the US on Monday, and can be picked up through all the usual import channels. And should be, since the last one was pretty much the best thing to happen to the Game Boy Advance.