So the big question: Will existing TI-83/84 programs run on the TI-84 C? I really hope so. Does anyone have any info on this? The new model also has less RAM, which will certainly cause incompatibility with current programs that use a lot of RAM. Why would they downgrade the memory size?

I can't answer all those questions without turning this into a giant Cemetech advertisement, but we do indeed have answers to all your questions. I wrote out two posts explaining why the 64KB (16-bit) z80 memory space means that even if the physical memory grew, without paging the VAT system they can't push much above 24KB (and had to fit more buffers, presumably, hence 21KB). We also know a few things about BASIC programs (they appear to be compatible command-wise, but homescreen programs relying on Output and almost every graphscreen program will look bad). We don't know much about ASM programming, but we're fairly sure it's some z80 flavor. In fact, one of TI's programmers, Pat Milheron, posted screenshots of the first TI-84+CSE game for our enjoyment.

Thanks! I was able to find the articles you mentioned (How did I not know about this until today? This is old news!). I'm sure people will make some cool new programs, yourself included (Doors CS C?!).

I was tentatively going to just call it Doors CS 8, but we'll see about that. It is certainly in the plans. Feel free to ask any questions there that you might have on the nitty gritty, and I will advertise this one URL with specs and photos: http://cemete.ch/tools/ti84pcse

We want pictures. :D

On the TI website it says: "Battery is expected to last up to two weeks".
I'm really stunned how energy expensive color displays are to this day. I mean, the batteries of my TI-83 last 10 month - a year.

A lot of the difference is not color vs. monochrome, but backlight vs. no backlight.