My apologies. I forgot that we were using bukkit for 1.4.7. We're on vanilla 1.5 now. Found the issue anyways. We had problems with 1.5 and switched back to 1.4.7 but not fast enough, apparently. People already had stuff like quartz, so when that player logged in, it tried to load the map with a large quartz block structure but had no info for that block ID, hence the null pointer.
Sorry, I'm a noob to this ticket system. Meant for it to be private until I fixed where I pasted the full log in the wrong place.