As seen in the statistic graphs, aprs.fi was only receiving about 60% of the APRS packets from the APRS-IS between 2011-11-20 23:40:40 UTC and 2011-11-21 08:25:57 UTC. During this time it was connected to a core server which apparently is not getting a full APRS-IS packet feed for some reason. As a visible result the packets of about 40% of APRS stations did not get to aprs.fi.
The core server operator has been notified and aprs.fi is connected to another server again. Sorry for the trouble.