As mentioned in an answer to the older version of this question, HomePlug AV uses AES-128. This paper suggests the cipher is used in CBC mode with a key that is changed at lease once an hour, so implementation flaws aside, the network can be as secure as as a WPA2-PSK wireless network.
One concern is the pre-shared key. The key is stored in each adapter and can be broadcast from any by pressing the pairing button, so anyone with physical access to one of the adapters can permanently breach the network. (Of course, the admins can securely change the key by plugging a computer directly to each of the adapaters, but I doubt anyone is willing to do this frequently.)
Another concern is the firmware. The adapters I own seems to allow arbitrary firmware updates from devices connected to its physical port. So someone with one-time access to the network may be able to upload a malicious firmware that defeats any security measure in the standard. (There are only a limited number of vendors who manufacture the chipsets, so it's reasonable to assume many devices have this feature/flaw.)
If you want to pretect against these, you can consider running a VPN or IPSec tunnel, in which case you more or less treats the connection as Internet.