Ok, to answer you properly:
Yes, i have been there to the top and also another section, to the side of Sierra Del Carmen (SDC) as we call it around here, FYI. There are also other members of the board who have gone to SDC and Maderas Del Carmen (MDC), but that area is completely off limits, since the land is iether leased or sold to CEMEX and that is a wildlife preserve. Very pristine,very unique, very difficult to go there.
There are no Outfitters to take you there (SDC) right now, only freelancers either from Boquillas del Carmen (The village right across from the river) and Norias de Boquillas. This village,actually has a collective deed up on top of SDC, which they used to have access by this closed canyon no longer accesible....until I go back again and pound to hell out of those boulders that are in the way. You can not go up there by ATV, it is next to impossible to squeeze them thru that bouldered up canyon and reach the top. You can reach it by horseback, which you can hire the services of the local ranch hands that take care of the land deed up on top.
If you do not habla espanol, your best bet is to hire a bilingual translator in Boquillas to take you to somebody in Norias and then hire another hand to take you up in a horse or a mule. The scenary is spectacular up there, very difficult to compare to anything i have seen, in pictures, here on the board. But if you ask if it is doable?, yes it is doable but not easy.
Also, to gas up for a trip up there, you have to fill up in Muzquiz and on from there is about 250 clicks, give or take a few kilometers of self reliance trip.
I would not even ponder the idea of going alone. A minimum of three 4x4 vehicles is higly recommended. To help each other out, should one get stuck out yonder.
I have been 4 times over there, if you care to get more informed, please read these two trip reports on the area.
This first Trip Report i did, way back when we only had black and white pictures and gun powder for a flash.

, and the
Last one when i checked out an old mine of the area.(Minas del Puerto Rico).
In short, i think you have two options:
1.- Hit the area as it is right now, you would have to cross over from Del Rio or Eagle Pass,Tx. Drive at least 8 hours to the area and 30 kilometers of dirt roads, with other vehicles to help out.
2.- Wait a bit and like everybody else here, and see if the Boquillas crossing will open back again in 2012.
But you will never regret it, should you decide to explore the area.