If you run a 32-bit OS installer on a processor in the Intel 64 architecture family, the combination of installed OS support for new ISAs and the processor's ISA capability (reported by CPUID) determine what ISA software developer and end-user can use.
AVX will require OS to have 256-bit YMM state support. Once the OS adds the necessary system level support, it can decide to create 64-bit distribution and 32-bit distributions. A 32-bit OS distribution with the required state support will allow software to use AVX.
The question of whether only newer 64/32 bit OS distribution will support AVX or any back-porting of XSAVE state support to older version is up to OS vendors/developers.
Irrespective whether you are using older hardware or older OS, you can get a head start with SDE from ISN's whatif download siteto start software development and have software ready when Sandy Bridge comes to market next year.