With the debut of theNikon D90, a third and mysterious L firmware module appeared below the A and B readouts on the. Until now, no one has really known what it was for.
Estimable former CNET freelancer and prolific book author David Busch took the time to solve the mystery and share the results in his blog: it's for the updateable lens database. Not the most thrilling news in and of itself, but interesting when you consider that it can also hold lens-specific distance information, which can theoretically improve performance of many of the subsystems, such as metering and Live View autofocus, in the D90 and all supporting dSLRs.
(Via Nikon Rumors)