Before every major release, salesforce.com lets its customers have a sneak peak at upcoming features. Not only do you get to play with some cool new functionality, but you also get to validate it against your own data and your own existing configuration.

The Sandbox Preview Window for Summer ‘10 is scheduled for May 14, 2010 through June 12, 2010. You can learn more about the preview window at
http://sites.force.com/ideaexchange/ideaView?c=09a30000000D9xt&id=08730000000H4CB

The instructions are somewhat complex as they are server specific, but in general:
1. DO refresh your sandbox prior to May 1, 2010
2. DON’T refresh your sandbox after May 1, 2010

If for some reason you don’t want the Summer ’10 preview in your sandbox then just break the second rule above. If you accidentally break it, then you’ll have to wait until Summer ’10 is live in your production org before being able to have it in your sandbox again.