In an OLAP cube, it is important that the cube can quickly figure out if a certain cell (or tuple, for the MDX fans out there) is empty or not. For measures, Analysis Services is keeping track of this. But for calculated members, the system has to evaluate the full expression, which slows down the query. Unless... unless we can tell the system that the calculated member will be empty (null) when another, real member is null. This is done via the non-empty behavior property of the calculated member.
It is common to check all your calculated members to see if this property is set (or there is a valid reason why not setting this property is OK). To help you with this, we develop a tool: NEBChecker. With this tool, you can quickly see all the calculated members in your live cubes which do not have a non-empty behavior set on them. The tool also shows you the definition of the relevant calculated members. Go to the U2U Resource center and give it a try!
UPDATE: NEBChecker doesn't exist anymore: it became part of the SSAS best practise analyser, wich you can download at the u2u resource center as well: