livin,
I don't think anyone will be able to help you gauge the level of interest, other than what you experience with additional communications and attending their camp.
The fact that the first contact you have received from a "fairly big time" DI program is midway through your senior year is a factor to consider, I think. Most of those types of programs are very far along, or have fully completed their class for 2008/09.
It is my thinking that neither you nor that school has anything to lose by your attending the camp. Maybe you are a player who has been overlooked and this will be a good thing for you and that program, if it is a good fit and you can play at that level. But a fairly high level DI program in CA calling based on a video and asking you to attend a camp in 2/08 has caution flags attached, for me.
It this were my son, with the new transfer issues for DI players, I would continue to focus on those schools who showed early and continued interest. If something else intervenes that works for you and is a good fit, that would be great. I would caution against changing where you have been focused because of a late entry with more glamour in terms of DI, especially with little to no knowledge or exposure to the coaches, or the program.
I would attend the camp but not expecting "I am there" as a top/scholarship recruit for the Fall of 2008. It is too far along for the type of program you are describing to be viewing you in those terms, IMO.