Here is what I just did on mine as a test. I only expanded mine 10megs, buit I didnt need to do it anyway. Without rebooting.
# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/RootVG-root
992M 342M 600M 37% /
# lvextend -L +10M /dev/RootVG/root
Rounding up size to full physical extent 32.00 MB
Extending logical volume root to 1.03 GB
Logical volume root successfully resized
# resize2fs /dev/RootVG/root
resize2fs 1.39 (29-May-2006)
Filesystem at /dev/RootVG/root is mounted on /; on-line resizing required
Performing an on-line resize of /dev/RootVG/root to 270336 (4k) blocks.
The filesystem on /dev/RootVG/root is now 270336 blocks long.
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/RootVG-root
1020M 342M 626M 36% /