In SSRS 2008 there is a KeepTogether property. Make sure your Properties window is visible. In your report table select the Group row. In the Properties window you will see the KeepTogether property (it is under Other, if your properties are sorted by category). I would assume it is the same for SSRS 2008 R2, although did not look. I could not find the same property in SSRS 2005.

Good luck,

Steve