A GPA in my opinion reflects who someone truly is, your average performance over 3-4 years. It shows how well someone can focus on the big picture and stay focused. Unreliable people can't do this and have a low GPA whereas someone with a 3.8-4.0 is really impressive to me.
I think your GPA can reflect a lot of things. Obviously, if somebody is down at 2.5 like I was, it says something about one's interest/ability in his major. And if someone pulling a 3.95, he is probably pretty rock solid regardless of his major. Between that, I think it's a lot fuzzier.
When I think about the best engineers I know, there is not a whole lot of correlation with GPAs. I know two 3.6's in computer science (summa cum laude at my UG). One is incredibly brilliant, and now works for the State Department. We talk with each other about new papers in the field, he has hobby projects on the side, etc. At work, he's a superstar. The other is just competent, but works really hard. I helped him through sophomore year CS classes, and it was a bit bothersome to explain some more complicated concepts to him. He got asked to resign from his first job because he couldn't pick things up fast enough, and is now comfortable in his second job because he's at a big company where there is a lot of training/support. If I was interviewing the two for a job where I work (small, startup-style company with NO handholding), I'd hire the first one in a heartbeat, and not hire the second, even though their GPAs would suggest similar levels of ability.
On the other hand, I know a guy with a 3.8 in aerospace, who switched from an ME major and still managed to graduate in 4 years. He's going to grad school now and I'm convinced he's gonna end up with a satellite or missile design with his name on it. His GPA reflects the fact that he's a fricking genius, and he's got the research chops to back it up.
I think what I'm trying to get at is that I see GPA as being a kind of tri-state thing. If you're at the top of your class, you've obviously got something going for you. If you're at the bottom, you've obviously got some issues. If you're somewhere in the middle, well, then I need more information. I'd rather have the somewhat below-median CS student who has two open-source projects on the side than the somewhat above-median CS student who just does schoolwork. I'd take the lit major at median whose published his work professionally over the top-1/3-ish guy whose just written undergraduate papers.