A family member of mine would like to be able to play old PSP games online. Unfortunately the device does not support WPA2, and my network is strictly WPA2 AES secured. What are the risks involved in operating a mixed WPA/WPA2 infrastructure, assuming that AES is forced rather than TKIP?
WPA isn't that secure, but then again WPA2 is no great shakes either. The main risk is that WPA's encryption is easier to break, and implementing WPA versus WPA2 would make it easier for an attacker to discover your key, which is the same for all devices presuming you are running personal mode, even if you can run mixed WPA/WPA2. WPA and WPA2 share some of the same structural vulnerabilities, so WPA2 isn't so much better that running WPA/WPA2 mode is an enormous step down in security.
Really though, part of your risk assessment should include likelihood, and what the potential impact could be. This sounds like a home setup, what is the likelihood that someone is going to pick your home's WiFi to crack, especially considering that there's probably some open or WEP-enabled networks that are easier pickings. And what is the damage potential if someone was successful? Is there anything in your home that someone would want to specifically target?
Most likely the actual risk is pretty minimal, but if you're worried you could enable MAC address filtering (although a skilled attacker isn't going to be slowed down much by that), or you could buy a second WiFi access-point (dirt cheap these days) and put it onto a DMZ network with no access to your main network.
Another option that you can use is to use a router that allows for more than one SSID (most routers that can run OpenWRD/DD-WRT support this), and set a different encryption to the "gaming" SSID.
Moreover you could use the router firewall to block the gaming network from accessing your network and only allow it to access the internet, so if that passphrase is hacked, the worst that can happen is that someone would be able to use your bandwidth.