Blackberry is a hugely implemented secure handheld based email client inside the RIM network. It can retrieve emails for you and notify you when you have new emails straight from your email server.
There's a Denial of Service potential in one of their devices, the 8100 Pearl (v184.108.40.206) which can be easily exploited. A fix is available, so if your organization uses BlackBerry and this particular device/model, please update to a patch.
More info: www.blackberry.com/security/news.jsp