So, for me I always recommend reboots of a BlackBerry Enterprise Server when there is a scheduled restart / failover of an Exchange or Global Catalog server (heck, BES needs both to work!). Unfortunately, not everyone can do this, and reboots / restarts / failovers aren't always planned.

BES isn't an Exchange cluster-aware application, so it can't effectively handle failovers like we'd all want it to ... well at least until Exchange 2010 (aka E14) is released and then with all MAPI...