Java EE, Linux Ubuntu Server, Jetty
Linux 2.6.32-22-server #33-Ubuntu SMP UTC 2010 x86_64 GNU/Linux

For some reason the JSVC process is at almost 300% (I think the max is 400%. It's a dual core with hyper-threading.)
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
22639 jetty 20 0 1509m 426m 13m S 295 11.0 778:00.27 jsvc

I don't like having it that high. There's definately something wrong. The uptime also indicate high load (app. 20 when it should be at 4)

root# uptime
14:10:23 up 100 days, 19:51, 4 users, load average: 22.75, 22.46, 22.31

I'm not too experienced with these things. I've tried reducing the hibernate-spring.properties
+++++++++++++++++++++++++++
-hibernate.c3p0.initialPoolSize=10
-hibernate.c3p0.maxPoolSize=25

persistence.xml
+++++++++++++++
-<property name="hibernate.connection.pool_size" value="20" />
Prevoiusly they were much higher.
Merely starting Jetty sends the jsvc-process to 300%.

How do I get down the load to a reasonable level?

It could be your application is just doing the stuff it's supposed to do. I'd suggest to use a profiler to inspect where it's spending the CPU time.

Shooting in the dark with random configuration settings requires too much luck! I doubt the pool settings have anything to do with it; to the contrary a common problem is people having a "slow" service while the CPU load is low; in that case some resource is blocking the full speed (like no enough database connections) but you seem to have the opposite problem.. using the CPUs you've got is usually a trait of healthy setups, why did you get them otherwise? Just make sure they are not spending power doing worthless/unintended stuff... no way to check that without a profiler or a crystal ball.