I don't see a way to display it in the system tree, but you can create a query. When adding fields, look at the "Computer Properties" section. I see a "CPU Serial Number" field, but not a system serial number. However, there may be enough other fields there that you would be able to identify the system.
We went through the execise of requesting professional services to incorporate this into the agent. It is such a simple yet valuable piece of information.
We gave up on them and went with a simple and free tool called LanSweeper. Lansweeper can inventory machines on the network and capture so much information including the serial. In addition, after a little configuration, I was able to check machine to see if the McAfee agent is installed, if the service is running, the dat version and so on.
Regarding the agent, I also put in a Feature request for this - I hoped to see it in the 4.0 versions - but no luck so far.