It all depends, really. It's preference. The microcontrollers on these boards aren't language specific, they take hex files which can be compiled from C language, or assembled for ASM. Thus, it's all about what the programmer in particular likes.
Myself, (and thus, the DV8's) I use Assembly. I know Curt (KM2) uses C. From what I know, I believe the Excal's board was done at a 3rd party engineering firm, so I would think it would be in C, but that's by no means definite. The only strong consideration when choosing the language (in respect to the particular chip used) is such: If you're using C, you must choose a compiler compatible with that particular chip line (KM2, Excals, 'Timi's, and Impulses use Microchip's PIC series controllers, DV8's use AVR controllers, and bushies, tribals, EM1's, and a lot of the others use 8051's). On the other hand, if you're using assembly language, well... If that's the case, it's pretty much a different language for each chip.
As I said before, I personally prefer assembly because of the more direct control over the exact commands performed, and the efficiencty. On the other hand, if you're looking to get something done as quickly as possible, C is MUCH more effective.
"...I question the contributions of some of the named inventors, specifically Billy Gardner and Adam Gardner..."
"...the evidence strongly suggests that neither Billy nor Adam could have invented what is claimed."
"As with the lack of any documentation of the Gardnrs' work, Billy and Adam Gardner's testimony regarding thier own contributions does not suggest the work of inventors."
U.S. District Judge Garr M. King, Re: Smart Parts' patent on electronic paintball markers