Sure but only if you architect it that way, which most people don't if they're using one big beefy server, because the whole reason they're doing that is to iterate quickly. It's hard to build something that can bust to the cloud while moving quickly.

Also, the biggest issue is where your data is. If you want to bust to the cloud, you'll probably need a copy of your data in the cloud. Now you aren't saving all that much money anymore and adding in architectural overhead. If you're going to bust to the cloud, you might as well just build in the cloud. :)