You can somewhat avoid this if you adjust your AD import option to only import active objects.  After the machines are retired, disable their computer objects in AD.

What made you realise the machines were reportig back in? I had this same problem and didn't find it for a while. We were running reports of sotware on machines that have reported in for the last X days and thought everything was fine, but because AD was not clean and we import from AD on a daily basis these machines were showing that they reported in, screwing up our numbers. To resolve this I put a check in clause into all of my reports so now they read off the last time the Altiris Agent checked into the NS.

SELECT c.[Name], a.LastConfig
FROM vcomputer c
    JOIN (
            SELECT
                    ResourceGuid,
                    LastConfig = datediff(dd,MAX([StartTime]),getdate()),         
                    LastConfigDate = MAX([StartTime])
            FROM Evt_NS_Client_Config_Request
            GROUP BY ResourceGuid                        
    HAVING datediff(dd,MAX([StartTime]),getdate()) < '21'
        )
            AS a ON a.ResourceGuid = c.guid
order by   a.LastConfig  

We only knew as we were trying to find computers with out of date virus program, when we were reporting we realised that these computers were in the list but no longer exist. I have checked AD and they were still in there - as there were only about 30 computers with the old version it was easy to pick out the ones that didn't physically exist.

I do like your above report and think it will also be useful