The way i did this using iTunes 9.2.1 was simply cloning the iTunes Music folder onto the new computer(i think Migration Assistant will work too).
This will effectively clone your iTunes library, and it wont matter which computer you plug it in to. Just remember to close down itunes and unplug any iDevices prior to cloning it. The iTunes versions should match each other.
It will however not keep your libraries in sync, which means you need to transfer purchases to both libraries if you frequently sync with both machines. The best solution in this scenario IMO is to use n macs for syncing, and a dedicated network-attached storage solution, such as a NAS, which hosts the "master" iTunes library. Then you can set up an automatic sync to your iTunes libraries locally on your Macs, and as long as you dont modify your libraries on more than one machine at any given time, your macs will always have the latest iTunes library available for offline usage, and as soon as you connect to your network they will sync any changes made since last, conflict-free.
Also, reportedly on iTunes 10 this one-library-per-device limitation is gone. You can (reportedly) play back music on your iDevices straight from iTunes 10 on any computer.
Havent read any specific info on device sync under 10 though.