The new server has been taken into use just a few minutes ago. This should maintain the high speed and performance of the aprs.fi service while the amount of users keeps increasing. Please drop a comment if I broke something on the way!
The servers are currently sharing load using database replication and DNS round-robin. The setup isn't really highly available or fail-proof. If one server breaks, the service will not function properly until I do corrective actions (remove the broken server from the DNS and possibly start the master APRS data collecting processes on the secondary server, if the primary dies). But at least that'll only take a few minutes to do, instead of a lengthy hardware repairing and backup restoring process.
There's also a third online replica of the database for the purpose of taking database backup snapshots on a third server (thanks ZFS on Solaris 10). So even if erroneous data is propagated over the replication and data is corrupted on all of the running database servers, I'll be able to roll back to an older snapshot and avoid losing all of the history data.
I'll continue working on automatic fail-over - it'd be nice if a hardware problem wouldn't cause much of an outage. I've done this sort of things before at work, so the plan is quite clear, it just takes some time to do it properly.