Borderlands should work in 3x1 portrait mode just fine, but landscape just isnt going to happen.
Yes, I finally got this to work. I found that Catalyst 10.5 and drivers are stable and don't crash borderlands in 3x1 portrait mode @ 3600x1920 resolution. Unfortunately, from my experience this same configuration on my computer using Catalyst 10.8 crashes Borderlands during startup. It seems they had the right formula in 10.5 but lost it with 10.8. Oh well, I'm content to have everything run under 10.5 :D