edit: this was originally a reply to the initial raid roster thread. since it is out of date, it has been split so that it can be maintained for historical purposes.
To clarify, a raid roster *is not* the final list of who will and will not raid. If the raid roster is around twelve members and only nine people show up, the raid leader will find someone else to fill the spot so the raid will run. This may be an infrequent raider. This may be another raider's alt. Since the Firelands is crazy hard tuned at the moment (for us anyway), the raid leader will generally favor someone who will make the run worthwhile for the other nine people who did show up. If only nine people show up on a regular basis, then the raid roster will be expanded to thirteen members and so forth.
A raid roster *is* the list of people being actively cycled into the raid. When more than ten people on the roster show up, one or two will need to be benched. They may be benched on a per boss basis (more common when the content is on farm) or on a per run basis (so people can go off and do other things). The people that are benched are rotated so that the entire roster is up to par -- par in terms of gear and par in terms of fight experience. In an ideal world, a raid roster would have exactly ten people because the same ten people always show up. Since this is not an ideal world, the raid roster is a number greater than ten (presently it is twelve). All twelve people are expected to be able to show up regularly. If you raid once a month, the raid leader is not going to bench someone who raids weekly just to make a spot for you.
If you expect to raid regularly, but are not on a roster, speak up. Don't just sit there waiting for a spot on a raid roster -- find out what it would take to get on one. Right now, there is a third raid group forming up. When that one fills up, we can always start a fourth.
If you expect to raid infrequently, but a raid leader passes you over for someone's alt, speak up. The usual reasons are reputation for standing in fire, poor grasp of fight mechanics, or low performance. The reason why raid leaders tend to favor a raider's alt is because they have a grasp of the boss fight mechanics (from their main). If you think you have that base covered by adequate preparation, speak up.
-HP