Adobe Lightroom's single license covers installing it on both your main desktop and your laptop. It even allows you to install one copy on a windows computer, and a 2nd copy on a mac.
How many computers are covered by a single license of Adobe Photoshop Lightroom 3?
Subject to the terms of the software license agreement, the primary user of the computer on which Adobe Photoshop Lightroom 3 is installed may install a second copy of the software for his or her exclusive use on either a portable computer or a computer located at his or her home, provided that the software on the second computer is not used at the same time as the software on the primary computer. Lightroom is sold as multiplatform software, which means it can be installed on either Mac OS X or Windows.
From Lightroom's FAQ under "Pricing and Availability."