Generally, you cannot. At least not easily. If you buy Apple's VESA mount adapter, it comes with a tool to help with remove the stand (you also need to undo some screws), but it is for the 24-inch iMac only, and it is not meant to be something you do constantly.
There are bags and cases that are specially made to carry an iMac. For example
http://mylugger.com/index.cfm?fuseaction=upgrades.products&subcat_id=1
I have not used any such products.