When it comes to bounce rate, there's no hard number you can use for a benchmark. It will vary all over the place depending on the site's content and purpose, as well as the quality and source of the traffic.
The best way I've found to use bounce rate is to work with the pages that have the highest percentage and try to see why the traffic is bouncing - does the page attract people to look further, or does it just answer a question? Is the traffic source somehow off - such as ranking well for a crazy query term?
Some A/B testing often helps at that point, and the goal is making the bounce rate lower than it was, but there's no specific number to target.
[edited by: tedster at 5:32 pm (utc) on Jul 11, 2010]