Can I clone my desktop Mac OS to my MacBook Pro?

The OS files on a Desktop and Laptop are different so I don't advise trying to run one on the other and vice versa.

If you just need to copy/synch your user files like Documents or Photos you could just synch/copy those only ?
e.g. Sync2Folders
http://tcfj.pagesperso-orange.fr/site/programmes/realbasic/SyncTwoFolders_i.zip

It'll be fine, so long as the OS you're trying to run is later than the one that came with your system.

I do it all the time at work. I have an external drive split into 4 partitions. Three are for diagnostic programs, the fourth is an emergency OS I can use to boot any system if I need to verify something and the internal HDD is shot. And it's nothing special either.

So, say for example the laptop shipped with 10.5 (and it's an Intel based system) and your desktop shipped with 10.6. It would work just fine. If it's the other way around, you may have problems. My new 27" iMac requires 10.6.3 at a minimum, so if I tried using anything less it would kernel panic at boot.

A simple time machine backup then restore via OSX install disk will be a 100% clone