been using jessie on older pis for a while with lessbian
With the old Pi it was not possible because the CPU only supported the ARMv6 instruction set, while mainstream distributions (Debian, Ubuntu, Fedora, etc.) all require ARMv7.
Fortunately the just released Pi 2 sports Cortex-A7 Cores which use the ARMv7 instruction set, which made this possible without any full rebuild.