Hi Ian,
The main work for supporting mongo 2.4 is making new mongo clusters for meteor deploy and migrating all the deployed apps to the new database. After that, we just have to update mongod in the dev_bundle to 2.4 so people get the new version in development mode.
I've been thinking we should migrate to a new mongo cluster anyway to take advantage of the new EBS optimized instances on EC2, and maybe the provisioned IOPS EBS volumes too. More I/O capacity could let us use fewer database machines.
No promises on timeline, though. Probably not in the next couple weeks, maybe sometime after that...
-- Nick