This looks like a useful bit of info I found on MacRumours forums

OK. Played around with disk utility and found a way to do this.

1) I partitioned my external HDD by using the GUID partition table;
2) Inserted my Snow Leopard install disk;
3) Created a new image (dmg) of the SL install disc on my desktop;
4) Scanned the new dmg file with disk utility (under images>scan image for restore);
5) Selected the new HDD partition and selected “Restore” tab in disk utility;
6) Selected the SL dmg file as source and dragged the hdd icon in the left menu to the destination field;
7) Hit apply.

Now I have the SL install disk on hdd if I need to reinstall or fix disk errors and/or permissions.