In order to understand the switching concept, one must alos understand the bridging concept. Bridging is allowing hosts in the same broadcast domain to communicate without L3 capability. A hub works much like a bridge where everyone in that hub is in the same broadcast domain. The technology got better and switches came along. along with switches came vlan concept. the idea with the switch and vlan are having a vurtual bridge, bridging ports that are in the same vlan. for example, ports 1 and 2 are configured to be in vlan 1 while ports 3 and 4 are configured to be in vlan 2. Hosts in ports 1 and 2 are in the same broadcast domain while ports 3 and 4 are in the same broadcast domain. you may look at it that ports 1 and 2 are bridged and ports 3 and 4 are bridged. this gives switches a greater flexibility to support many vlans (bridged ports) unlike a bridge device that pretty much act like a glorified hub. Now, since they are port that are bridge, you cannot put an ip address on the interface, this would make that port L3 port and no longer an L2 port or part of a bridge/vlan. so, if you think of a "switchport" as being part of group of ports that are bridge together, then it is easier to understand why you cannot put an ip address on a "switchport".
link from Devang have a good explanation of the vlan concept and this link might help understand it as well:
http://www.cisco.com/en/US/tech/tk389/tk390/technologies_tech_note09186a00800a7af3.shtml#vlans
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