In April 2005, Iridium 16 (24841, 1997-030F) was removed from the operational constellation, and subsequently Iridium 86 (25528, 1998-066B) took its place in the operational constellation.
This was first highlighted by Rainer Kresken as Iridium no-show on Friday, May 13, 2005 on SeeSat-L -see http://www.satobs.org/seesat/May-2005/0160.html. Mike McCants provided the explanation of the changover of satellites in a reply - see http://www.satobs.org/seesat/May-2005/0164.html.
Up to and including April 10, 2005, Iridium 16 appears to been a normal member of the constellation. On April 11, 2005, its orbit was slightly lowered.
Iridium 86, the spare for orbital plane 5, seems to have remained in the spare orbit until April 19, 2005, and was then raised to operational altitude over the next few days to occupy the slot formerly occupied by Iridium 16.
This now left orbital plane 5 without a spare satellite.
Whether an attempt will be made to move a spare satellite from another orbital plane, in readiness for any future failure in orbital plane 5, remains to to be seen.
Update: As of late June 2005, Iridium 98 is manouvering, and may be in the process of being moved from orbital plane 4 to orbital plane 5.