Log in to the Mac Developer Centre with your developer account (requires $99 Mac Developer Program membership), and request a code that will allow you to download the installer from the App Store.
Ok... how about 'ileagal'? i am not using it for every day use, just litterally creating a partition on my HDD then installing it there just to have a little play.
Lion isn't "stable" as of yet...so if you do want to test it, go ahead and get a developer account. 99/yr is reasonable given the amount of access to apples OS's.