Gnoming has become a bit of a lark in recent years and as a temporary, "on loan" bit of fun, it can be entertaining. Just don't do this to a narky neighbor who notices when anything at all goes missing or is out of place!

[edit] Steps

Get a group of friends together. All of you must swear to secrecy!
Find a neighbor's house with a garden gnome. Make sure your neighbor is a good sport or away for the weekend.
If not, just try this. Get some charity leaflets and knock at the door(alone).if they answer, just offer the leaflets and make a quick get-away.(alternatively just pretend this was left for them).
if they don't answer grab that gnome!
Drive around town doing random things with the gnome. This might include shopping, ice skating, having lunch etc. Do anything that comes to mind and looks like it would be a great photo opportunity for the gnome.if you have any town-specific landmarks(town hall, monument etc), take pics of him near that too.
Take pictures during each activity. Put them in a small photo album, or scrapbook.
Return the gnome to the porch of his house with photos. Make sure the neighbors are home or you'll miss the fun that ensues.
Ring the doorbell.
Run away. You and your friends should remain hidden but try to be somewhere that you can observe the neighbor's reaction. If your neighbor is a really good sport, ask them about it a few days later.

[edit] Tips

It is a good idea to borrow the gnome at night or during a time of day when the resident of the home is not present.
it is best to do the actual 'borrowing' alone or with one friend, otherwise nosy neighbours will suspect. just walk up discreetly but confidently, and take the gnome.
if you are in a LARGE group or know the neighbour well, or if the neighbour is cranky, don't ring the doorbell!
you don't have to put the photos in an album if you don't think the reception to said photos will be good.