Here's the deal, from my perspective:
First and foremost, you can always get another battery for your DS. Check out dealextreme.com if you dont' believe, or, hell, eBay. I'm impressed you kept with just the original incarnation of the DS, and actually encourage you to ride that horse until it dies.
Secondly, keep in mind that the 3DS will not be available until 2011, and even then, only in Japan at first. At the time, we have no real idea of backwards compatibility, availability, or even what the thing could look like. All the hard work that's being done on the homebrew scene (both for DS and DSi) may need to start from square one after this sucker drops. I'd even go so far as to say that, if you're running a flash card now, you will be up a creek if you make such a drastic jump in systems.
Thirdly (and lastly) you may be dreadfully unsatisfied when the 3DS arrives. I'm certainly not holding my breath for world revolution: look at the horor that 3D tv is unveiling. The only good thing that's come out of that abominable move is that I can buy a new HD tv this summer for cheap. While you wait and see if the messiah is coming in a new 3DS package, the DSi is getting cheaper and cheaper, and the DS Lite is nothing.
My vote? Get the DSi. Get it now, enjoy the hell out of it, and wait and see what Nintendo does with 3DS 2.0. After this wild success ride, I feel that Nintendo has turned into Apple, and we can expect another incarnation of the 3DS within a year of it's initial release.