Thanks again everyone!
Here's how i got the color (and it certainly wasn't an overnight process!):
I started lightening my hair using Clairol Luminize, in Clear. It's a gentle lightener that very subtly lightens your hair color. The first time I applied it was in January of 2008. I re-applied it every 2 weeks for about 2 months, and after 2 months, I applied henna on top of it. The result was a very deeeep brownish red, nowhere near as bright as the shade it is now.
At that point, I wanted a more dramatic color result. I stopped using the developer that came with the Luminize kit, and switched that for Clairoxide Volume 30. (the luminize developer is a volume 10, but i wanted a more powerful lift). I applied that, and immediately saw a difference. It lifted about 2-3 shades each application. I applied that once per month, all over my hair, for about 4 months, until i finally got a shade that i loved. I then did another henna application. The result: hair that is slightly darker than what's pictured in my siggy.
At that point, i stopped applying the luminize all over, and continued to touch up every 2 months just at the roots, with the 30 volume developer + a bottle of Luminize in Clear. Every so often, I'll apply it through all of my hair to brighten it, and then henna on top for the bright red result you see in my siggy.
So basically, this color took me over a year to perfect.
I should mention that this is with light reflected. Check out my fotki for how my hair looks in different shades of light. Normally, it's much deeper than how it's pictured.
I've been a very happy redhead.
This thread did come as a total surprise; i plan on indigo-ing on top of all of this for a blue black look in the near future! Y'all caught me just in time!