NOTE: Unlocking the bootloader will automatically do a factory reset, wiping all your data and settings (including any files you have in the internal sdcard, I don't think it wipes the external sd card, but I didn't have it plugged in at the time I did it).
First you need to install Lenovo's LeTools... You can download them from here:
Next, you need to reboot your A2109 into the bootloader, there's two ways to do this:
1) If you have adb working, you can type 'adb reboot bootloader'
2) Power down the tablet, power back on and immediately tap the Volume Down button.
If you've done this right, A2109 should have this on the screen:
Starting Fastboot USB download protocol
At this point, Windows won't recognize your tablet, but you just need to force it to use Lenovo's Fastboot driver, which was installed with LeTools. These instructions are for Windows 7, but should be similar for XP or Vista:
1) Goto the Device Manager
2) You should have a device called 'Fastboot' with a yellow exclamation point in the category 'Other Devices'.
3) Right click on 'Fastboot', and select Update Device Driver.
4) Choose 'Browse my computer for driver software'
5) Choose 'Let me pick from a list of device drivers on my computer'
6) Locate the Category 'Lenovo ADB Devices' and double-click it.
7) Choose 'Lenovo Fastboot Device' and click 'Next'.
8) You might get a message 'Installing this device driver is not recommended... blah blah blah", just click 'Yes'.
9) You'll need to unplug and plug back in your tablet once before fastboot will work.
You will need to use Lenovo's version of fastboot.exe, NOT the one in the android sdk. It is located in:
C:\Program files\Lenovo\LeTools for 32-bit Windows or,
C:\Program Files (x86)\Lenovo\LeTools for 64-bit Windows
Open a Command Prompt and type:
cd "\Program files\Lenovo\LeTools"
cd "\Program Files (x86)\Lenovo\LeTools"
You should see something like this if it recognizes your device:
Now to unlock your A2109 (remember this will wipe your device!!!), type:
fastboot oem unlock
On the screen you will have two choices, 'No don't unlock' and 'Yes unlock it'. Use the Volume Down button to select 'Yes Unlock It', and press the Power button to select.
Your tablet should now be unlocked.
(Thanks to mcl630 for providing this information).