I'm currently running Mint 5 and I'm thinking about upgrading to the new x86-64 version of Mint 5. I remember reading about a feature a couple years ago online. Some x86-64 Distros had a feature called multi-arch(itecture), where the kernel, HAL, the package manager and API calls were 32-bit/64-bit aware - Using 64-bit applications when possible, but switching to 32-bit compatibility mode and using only 32-bit programs if a program couldn't be utilized in 64-bit.
I remember a symptom of this feature was that there were /Opt-32 and /Opt-64 directories as well as a /Usr/bin-32 and /Usr/bin-64. The result of this was that you could mix 32- and 64-bit applications with out fear of incapability. I remember at that time there was no multi-arch in Ubuntu because there was still programing that needed to be done on APT by Debian to make this work. Is that finished, still being worked on or did I come to misunderstand something?