Luckily for you, high X.org CPU usage doesn't mean you have a bug in X, so this problem is likely something you can solve.
X is a server, which means it responds to client requests. If a client has lost its mind and is making excessive X calls (such as if it locks itself into a graphics updating loop), it can make a lot of X calls. This shows up as a high X CPU load.
So, the thing to look for is to identify which client program is causing the load. Often it is a program you don't realize is even running - perhaps a panel indicator or background task or service. Look through your process table for user processes that look out of place, and try killing them one by one until the load goes down. Then you know what caused it.
Alternatively, log into a guest account; verify the load is normal there. Compare your processes in the guest account with those in your regular account, and go from there.
While high X cpu bugs are not at all unusual, I have not seen many reported in oneiric, so I might guess that the faulty process is some unusual program not installed by default; perhaps something you manually installed previously that got automatically upgraded to a new version that has a bug in it? Anyway, good luck in your hunt.