Using a Macbook Pro alongside an iMac If I have a 27" iMac, I understand I can use the display as an external monitor for my Macbook Pro. Presumably I can use the bluetooth keyboard as well as an external input?

Can you still run the iMac (and maybe instantly switch between them?). e.g. if I'm working on the Macbook using the iMac as a display, can I still listen to music on the iTunes library that's on my iMac, or switch to it to read a file (or download something from iTunes etc)?
Using your mac in target display mode would waste it's potential as a more power computer than your MacBook. Try an app like Teleport. It lets you network your multiple Macs together so that a host Mac's mouse and keyboard can be used to control other Macs. It works very similarly to multiple displays on a single machine, and even the clip board is shared between the two! The only issue is that you can't drag windows between the two displays (which are actually running on two separate computers), but at least both Mac's power can be used.