2.00 TB of PV "not usable"?

[I've searched google and the list archives for the answer to this question, but without success.]

I've got a 3.18 TB PV member of a VG, and everything seems to be working fine. But, I noticed that the output of pvdisplay for my PV shows:

...
PVSize 3.18 TB / not usable 2.00 TB
Allocatable yes (but full)
...

My question is: should I be concerned about the "not usable 2.00 TB" part?

Thanks,

-chris

p.s. kernel is 2.6.9