I have a somewhat similar tale to tell. I suspect it's more a problem with Optifine than it is the texturepack I use, but I do know that Optifine_1.4.6_D5 doesn't play that nice with Ultimate pack 1.0.1, at least on my card (ATI HD3450, using fglrx driver) and OS (Ubuntu 12.10). For example, I start up the launcher, go into Ultimate and make sure that Optifine is in the JarMods folder, then I start Ultimate up, go into Texture Packs, make sure default texture pack is selected, then click Done. Once that happens, the console shows the client dying from a failure in the external renderer in libdri.so. Wonderful.
It also happens if I change texture packs to faithful32 (the FTB Ultimate pack variant) before I install optifine, then I install optifine, restart ultimate, start up my singleplayer world, and open a iron chest. Boom, dead.
Wonderful.
(Post 4)