[CentOS] Rescan harddisk size without rebooting

Hi list,
I have a SAN attached to a CentOS 4.2 server. I have expanded the size 
of the virtual disk within the SAN (by adding a new HD to the disk pool) 
and need CentOS to see the new size (CentOS see it as /dev/sdb). I'm 
using LVM. Do you  know a method for the Volume Group to see that one of 
its harddisk is now bigger, without rebooting (it's not a problem with a 
reboot but since the LDAP directory is on this server, it is problematic).
Thanks.
kfx