If you are asking whether the physical volume(s) that make up the volume group can be resized, then the answer today is No. Someday, the answer will be Yes, but the lvm pvresize command is not currently supported.
If you want to install Debian in the lvm, then minimize the FC3 logical volumes and create logical volume(s) for the Debian installation. It's covered in the LVM Howto (see Chapter 11 “Common Tasks” http://www.ibiblio.org/pub/Linux/doc...tml#commontask
You can create the logical volumes for Debian manually in advance or you can create them during the Debian installation.
You will need to use the commands resize2fs and lvreduce (or lvresize) to shrink the FC3 logical volume(s).