Install rEFIt. It's a bootloader that installs on your OS X partition. In the file
/efi/refit/refit.conf, there is a section that reads:
# List legacy options first. When enabled, legacy BIOS based boot options
# (e.g. Windows, Linux via LILO or GRUB) will be listed first. This is
# intended as a quick fix to change the default boot choice until full
# configurability arrives.
# Set the default menu selection. The available arguments match the
# keyboard accelerators available within rEFIt. You may select the default
# loader using a one-character abbreviation for the OS name ("M" = Mac OS X,
# "L" = Linux, "W" = Windows). You may also specify a digit between 1 and
# 9, in which case the Nth loader in the menu will be the default. You can
# also select a rEFIt tool entry ("S" = EFI Shell, "P" = Partitioning Tool,
# "U" = shutdown). This is intended as a quick fix to change the default
# boot choice until full configurability arrives.
legacyfirst or setting
default_selection W should work. I have not tried this although I use rEFIt without any problems. In the same file, you can change the
timeout 20 line to something shorter as well.