Hi Seb (and any others who might be listening) -
I've racked my brain (and the internet) over the past three days trying to get this to work. As described above, the only combo of optifine/shaders that seems to work for Hexxit (which has really breathed in new life to Minecraft for me) is the 1.5.2 ultra 3 optifine and a modified GLSL shaders mod - the problem is that ladders/vines do not work, a pretty-well known bug that's throughly documented with that version of optifine. Of course, upgrading Optifine to the next version (version 5) allows ladders to be climbable, but for the life of me, I cannot get the shaders to work at that point (nor with any other version of optifine/GLSL and Hexxit).
If anyone has gotten optifine/shaders *and* ladders to work with Hexxit, could you please consider uploading your modpack.jar file (or emailing is fine as well, trying not to be selfish)? In return, you will have my undying adoration (along with a bevy of gratuitious emoticons)...