Now I'm using XP Pro instead of the desired Windows 7 beta, I can now see the boot camp partition.
It's a weird behaviour because my keyboard's Alt key (Option for Leopard) doesn't work anymore. It worked once when I switched from Windows 7 to Leopard but not in the reverse direction.
I'm wondering if I need to invest in this
There is a very nice walk-through here but I can't see other people having similar problem like mine. My problem, which is that the boot camp partition installed with Windows 7 has gone missing, appears to be a known issue after my search in the forum see here, here and there.
May be I'll try again when Windows 7 RC is in the wild.