Mint 11 & Mint 12

[#1]

Currently running Mint 11. Mint 12 can be run from the disk but if I install it on an existing Dual boot system running Mint 11 & Windows 7 what happens ? Will it recognise these other OS and just make a new partition and add it to the Start up menu or is it likely to wipe one or both existing OS ?

[#2]

Are you planning on sharing your /home between the two mints?
If so, Mint 12 will be likely to upgrade the contents of your hidden files in /home which hold your personalised configuration.
This could upset Mint 11.
You should share any swap partition between the Mints.
Why not try running Mint 12 as a virtual machine in Mint 11 - if it's just for a trial run.
I've recently tried sticking Windows into a VM in Mint 12 and it seems to work quite quickly - but with no USB access.

[#3]

You can run them side by side, I do (I have 5 Linux versions running on this machine)
Make sure you have at least 10Gb free space and install it into that, I would recommend using a NEW user name for it as Fox said it will upset your current setup if you use the same user.
When you install it, choose custom, and install it to a new partition, it will automatically pickup the swap partition, and you can point it to your existing /home partition or let it just use the new partition for everything (recommended if you just want to play not use it, as you can use your existing username if it doesn't use your shared/Mint11 home partition.)
@Fox you need to install the non-OSE version of VirtualBox to use the usb and have the guest addons installed. I use it and my XP sees my usb devices (that I want it to)

[#4]

I agree with the last post.
I have three Linux distros on an external HDD and Win7 on the main HDD. Whenever I have installed a new distro, Grub has invariably found all the previous  distros on boot-up.
I always ensure that Grub in installed to the MBR on the internal drive.