Here's a little bit of techincal-minded speculation:
They got rid of the GBA Port, but i believe that address space is now beeing used for a bunch of stuff. Since all of the I/O on the DS is connected to the ARM7, i believe the now "free" connectors are beeing used for the 2 cameras, and for the SD controller - this is pretty much a given. Also, they talked about incorporating the web browser into the DSi, and as we know from the Opera browser, that needs additional RAM. This used to be provided by additional RAM on the GBA bus, but i guess thats gonna be built in now. So i take a shot that the DS will have an additional 8MB of DRAM (since the Opera expanion pack had this much - so thats my basis).
However, i doubt the developers will be allowed to use that RAM, because of 2 reasons.
#1 The bus is slow compared to the 4MB thats used as Main Ram. Just ask the guy who's developing the Quake 2 DS port.
#2 Nintendo won't license a game like that, fearing it will alienate the userbase. Also, no developer would do that anyway, since theres a lot of DS/DSLites out there, and not a lot of DSi.
As for the rest of the specs? They SHOULDN'T be upgraded, on a simple basis - they don't have to. For all they want to do, the hardware is perfectly capable. If one would *really* want, he can code this up on a DS/DSLite, didn't DSOrganize download homebrew to the SD card via DLDI and the could run it? So there. I was kinda wowed by the PhotoBooth-esque camera effects, but i guess thats just some clever ARM coding.
P.S. This is just pure speculation, and should be treated like so. However, compared to most speculation here I'm trying to base it on technological facts