This depends on how you have LANDesk is configured. Here is an article that describes how to automagically delete old inventory records http://community.landesk.com/support/docs/DOC-1188
Software changes - check this thread http://community.landesk.com/support/message/21301 There are options to do this natively in LANDesk and with third party products. Also check out Jack's solution on dropped packets.
Thanks for the reply.
If a client was removed from landesk but then came back on the network (like a portable computer) would it automatically re-appear in landesk?
Yes as soon as the client kicks off an inventory scan. Also if you have the gateway appliance, out of band mobile devices will never drop off since they report back through the gateway.