Quick iSCSI Overview
iSCSI is useful for exporting block devices via the network. A couple of use cases are as a replacement for a SAN (good for remote performance storage for databases, works well with lots of operations on small files) and as backing stores for Virtual Machines (especially so as just about any virtualisation platform (tested with VirtualBox and KVM, assumed to work with VMWare Server on linux) as well as a nice way to mirror storage between multiple servers for fast access to block level data on one server when another fails (works well with mdadm)
iSCSI Basics
The idea of iSCSI is you have a local server, lets call this server1, and a remote server, server2. They both have physical disks managed however you like (in a server situation a well tested reliable configuration is LVM aggregating groups of raid5's or raid6's assembled by mdadm, see releveant guides (note to self you should link those!)
Scenario 1: server1 is a san exporting blocks to server2
You will need iSCSI enterprise target on server1 to export stuff to server2, this is called exporting targets!
server2 will then connect to server1 using the open-iscsi package, which creates local /dev/sd* devices
iSCSI Enterprise Target
To get started with iscsi you have to install iscsitarget (tested with 8.10, 9.04)
sudo apt-get install iscsitarget
argh connection dropped out and i lost a lot 
will add more later