OK Sam
Here is a link to an article that explains what I have done
http://http://osxdaily.com/2011/11/17/install-run-mac-os-x-10-6-snow-leopard-in- virtual-machine-on-lion/
My experiences so far.
Firstly I have used VirtualBox for a couple of years running Windows VM's (virtual machines) for web developement testing. This has prooved very useful and the software is well thought out and reliable and there is good documentation.
I now have Snow Leopard running as a VM as well. This means I can jump between Lion and SL without having to reboot. Also with file sharing you can move stuff between machines easily.
What this means is If I want to work on a 2000 line document I can do it with scroll arrows and nice big easy to use scroll bars.
Also its a way of running any old Rossetta apps if you still need them.
The other advantage of this is when Apple release a new software upgrade you can try this out in a VM first.
If only I had thought of that before upgrading to Lion.
Drawbacks
1. You need a fair bit of memory. Although VBox recommends 1GB allocated to SL it works better with 2GB I am using a 20 inch Imac with 4GB and in practical terms I think this is probably the minimum.(of course you are only using this when your VM is running)
Also this does use disc space (20 GB for SL plus Vbox itself).
2. The maximum screen resolution supported for Mac OS X is 1440x900 so the Virtual box full screen feature will not on a larger screen. Actually for me this isn't really a drawback as its easier to move between machines and 1440x900 nearly fills a 20inch screen.
3. I find the mouse pointer sticks in the VM sometimes if you have the mouse integration feature turned on. If its turned off you have to press the LH option key to release the pointer from the VM.
A Few Tips
Follow the Instructions very carefully and make sure you reformat the VDI in the SL intaller process using the Utilities menu
When you set up SL as a VM I found the following settings worked for me after 3 attempts. so this is not a quick process it took me half a day.
1. Select Mac OS Server (64bit). you can probably also use the other setting just Mac OS Server
2. Select a Fixed size VDI 20GB this is important and is not in the article instructions a dynamic VDI will not work for SL
3 It actually took about an hour to install.
Hope this helps