See #221...it's an old Memogate story I found about supposed computer hacking by staffers in 2003. Would it be possible to get them in that manner?
The easiest way to get IM cache/history files is right off of the computer used to IM. Next would be from the ISP (In this case, AOL) server. Anything's possible, but the firewall and any monitoring software wouldn't be accessable through unsecured logins or areas of the network. Usually only the IT department has access to that.