Actually, merl1n isn't precisely correct.
If you are copying from say, your iMac to your Macbook Pro, you'll quickly find that even if you connect as your host name on your bigger machine, you won't be able to copy those files until you share the folder (AND ALL SUB-FOLDERS). This is a real PITA about the way Apple tries to implement security and fails miserably. Simply checking "File Sharing" will not accomplish this, in Snow Leopard that is.
I'm open to a solution, by the way, that contradicts this. But I have an iMac and a brand new MB Pro, both running Snow Leopard, and setting file sharing did not allow me to copy across machines unless I shared all the way down the tree, manually. There is no way to share a directory and have it share it's subfolders automatically. (Changing permissions won't work either).