grub2-mkconfig omits part_gpt module

Do you by any chance have one disk that uses GPT and another disk that
uses the traditional DOS partition table format? (A partition management
tool would probably tell you, if you don't know offhand - e.g. 'sudo
parted -l').

This is correct - parted -l lists /dev/sda (and SSD) as a gpt partition table and /dev/sdb (also an SSD) as an msdos partition table.

This bug was fixed in the package grub2 - 1.98+20100614-1ubuntu1

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grub2 (1.98+20100614-1ubuntu1) maverick; urgency=low

  * Resynchronise with Debian:
    - Add btrfs probing support, currently only in the single-device case
      (LP: #450260).
    - Insert partmap module in prepare_grub_to_access_device to handle
      cross-partmap setups (LP: #451585).
    - Fix verbose error output when device-mapper isn't supported by the
      running kernel (LP: #526045).
    Remaining changes:
    - Adjust for default Ubuntu boot options ("quiet splash").
    - Default to hiding the menu; holding down Shift at boot will show it.
    - Set a monochromatic theme for Ubuntu.
    - Apply Ubuntu GRUB Legacy changes to legacy update-grub script: title,
      recovery mode, quiet option, tweak how memtest86+ is displayed, and
      use UUIDs where appropriate.
    - Fix backslash-escaping in merge_debconf_into_conf.
    - Remove "GNU/Linux" from default distributor string.
    - Add crashkernel= options if kdump and makedumpfile are available.
    - If other operating systems are installed, then automatically unhide
      the menu. Otherwise, if GRUB_HIDDEN_TIMEOUT is 0, then use keystatus
      if available to check whether Shift is pressed. If it is, show the
      menu, otherwise boot immediately. If keystatus is not available, then
      fall back to a short delay interruptible with Escape.
    - Allow Shift to interrupt 'sleep --interruptible'.
    - Don't display introductory message about line editing unless we're
      actually offering a shell prompt. Don't clear the screen just before
      booting if we never drew the menu in the first place.
    - Remove some verbose messages printed before reading the configuration
      file.
    - Suppress progress messages as the kernel and initrd load for
      non-recovery kernel menu entries.
    - Keep the loopback file open so that subsequent changes to the "root"
      environment variable don't affect it.
    - Change prepare_grub_to_access_device to handle filesystems
      loop-mounted on file images.
    - Ignore devices loop-mounted from files in 10_linux.
    - Show the boot menu if the previous boot failed, that is if it failed
      to get to the end of one of the normal runlevels.
    - Handle RAID devices containing virtio components.
    - Don't generate /boot/grub/device.map during grub-install or
      grub-mkconfig by default.
    - Store grub-pc/install_devices as persistent device names under
      /dev/disk/by-id/.
    - Change priority to optional to match the priority of grub.
    - Don't display "GRUB loading" unless Shift is held down.
    - Adjust versions of grub-doc and grub-legacy-doc conflicts to tolerate
      our backport of the grub-doc split.
    - Fix LVM/RAID probing in the absence of /boot/grub/device.map.
    - Look for .mo files in /usr/share/locale-langpack as well, in
      preference.
    - Don't run /etc/grub.d/README, even if it somehow ended up being
      executable.
    - Make sure GRUB_TIMEOUT isn't quoted unnecessarily.
    - Probe all devices in 'grub-probe --target=drive' if
      /boot/grub/device.map is missing.
    - Adjust hostdisk id f...