Erm... that's the voicemail notification light.
Just to add a note to the info from my friend Aaron (+5) that is also the light that flashes as part of the visual ringing notification.
MWI - Message waiting indicator for Voicemail
Refer section "Physical Description"
Which is one of the good learning I had in the recent past, till this time I was under the impression that both of them work independently, never recognized the tiny lens system though.
You made my day Rob, Thanks