So I downloaded a collection of viruses available from here: Index of /comp/virus, just to test the AV engine of Comodo’s IS suite only to find I had to extract the tar archives (using 7zip) first before most of the viruses were found (those in the zip archives were detected immediately on a manual scan). All viruses were detected when I extracted them from any archive by the on-access scanner.
Of course this is not a serious bug as a virus would first have to be extracted before it could infect a user, but if you scan zip archives (and possibly cab and other formats too, I have not tested that) then why not tar?
More information about the tar format: