Question: I bought a 240 GB SSD to replace the 500 GB primary HDD in my new MacBook Pro.

How do I clone the current Lion system image on my 500 GB HDD to the SSD? I want to clone the entire drive not just copy data.

I can only have either the HDD or SSD in the MacBook Pro at a time. I have an external 500 GB USB drive that can I use if necessary. I've seen suggestions on using Carbon Copy Cloner but I'm not sure how to proceed with only one disk in the MacBook Pro at a time.

Answer: 
So you can keep using your old 500GB HDD I would buy a cheap enclosure for it. They can be had with USB 2.0 for as little as $10.

Concerning the Software-Side, neither SuperDuper nor Carbon Copy Cloner are bad. Personally I, however, prefer Apple's own Disk Utility.

Simply boot from your Install DVD or Restore Partition (if you use Lion), holding down the ALT key while you power on your machine.

Both the Install DVD and the Restore Partition give you access to Disk Utility.
There simply hit the Restore tab and drag and drop your Source an Destination volume accordingly.

If you do not want to buy a new HDD enclosure simply clone your internal HDD to your spare external using the aforementioned way. Then replace your Book's HDD with the SSD. And clone everything back to it.
SuperDuper <http://www.shirt-pocket.com/SuperDuper> is also an awesome back-up product and can easily clone a drive making a bootable backup. You can use it for free for what you're trying to do, but it's worth every penny for the extra features.