Arg0n

No idea. Try using other builds. Tweak the settings, see if that makes a difference. I'm using the Ultra shaders and not having any trouble unless I put down too many microblocks.

Make sure your detail level is set to Fast. If it's still crashing, try disabling the shaders and logging in, then moving away from the chunk in question (preferably into the wilderness far enough that the chunk doesn't load anymore), and then activate the shaders and relog in. If it doesn't crash for a few minutes, but then crashes if you go back to the chunk you were in when it did crash, it could be some other block in Tekkit causing it, or someone might have used alot of microblocks underground. Even if you don't actually see them, underground blocks still get rendered by Minecraft.

Update: I think I've figured out what causes GLSL to crash in Tekkit. I think it's the miniblocks (when you cut up certain blocks with saws). If there are too many in one chunk, GLSL crashes when it tries to render the chunk. I think this is the case because GLSL crashed when I was making a large floor out of quarter blocks and then was stable again after I removed the floor using a non-GLSL build. As long as I don't use more miniblocks in that chunk, I can run GLSL without crashing.

I was able to get Sonic Ether's GLSL shaders working with Tekkit just fine for the most part. There's just two areas that cause me to whitescreen when I go to them. One is a large volcano base I built with my friends. The other area is a big volcano with nothing built on it. So far that's it. I've flown around for miles in various directions and been through the Nether without any whitescreen crashes, but once it starts loading my base or that one particular volcano, it whitescreens. I wonder if it has to do with volcanoes? I'm not sure because I found several volcanoes when I was flying around and didn't crash. I don't think it's biome related either since I'm fairly certain I went through every biome on my test flight. My initial suspicion was that it had something to do with some kind of machinery, but I've visited a few other player's bases so far, and they have machinery running, industrialcraft stuff as well as buildcraft stuff and pipes. I could also observe working redpower pipes. No crashes there. I'm at a loss to figure out what the cause is. Any ideas? It may be wishful thinking, but if I can find the cause maybe I can make a fix, or at the very least the information could help someone with more modding skill make a fix. Note that I'm using the 1.2.5 version of the shaders since Tekkit is currently using Minecraft 1.2.5. In the meantime, it seems the best option for those who want to enjoy Tekkit with these shaders is to just have two versions of modpack.jar. One with the GLSL stuff modded onto it and an unmodded one. If you run into an area that GLSL causes you to whitescreen on, swap modpacks and reconnect. When you're out of that area and want the pretty visuals again, swap back to your GLSL modpack. Not convenient, but it works.