Many of the major applications include a license which allows you to run a second copy of the software under specific circumstances.

Microsoft Office for the Mac can be installed on a desktop and a laptop system and this is legal per the license agreement that comes with the install CD.
1, "Grant of License":
You may install, use, access, display, run, or otherwise interact with ("RUN") one copy of the SOFTWARE PRODUCT, or any prior version for the same operating system, on a single computer, workstation, terminal, handheld PC, pager, "smart phone", or other digital electronic device ("COMPUTER"). The primary user of the COMPUTER on which the SOFTWARE PRODUCT is installed may make a second copy for his or her exclusive use on a portable computer.

Adobe End User License Agreements (in PDF of course) for their recent products is available for download here: http://www.adobe.com/products/eulas/main.html

The EULAs for the Creative Suite line, e.g. PS, ID, and Ill, (all governed by the same 'CS_combined' agreement) and Acrobat 6 both contain item #2.4, which addresses a user's installing the app on more than one of his/her machines. Wording is slightly different but the point seems to be the same. From the CS agreement:
2.4 Portable or Home Computer Use. The primary user of the Computer on which the Software 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 the Software on the portable or home Computer is not used at the same time as the Software on the primary Computer.

(For other Adobe apps, the item number may be different.) Of note as well is the specific mention/alternative of home computer (along with portable) in the above. Photoshop 7's agreement for instance only makes mention of a portable:
2.4. Portable Computer Use. The primary user of the Computer on which the Software is installed may also make a second copy for his or her exclusive use on a portable Computer provided the Software on the portable Computer is not being used at the same time the Software on the primary computer is being used.

At least for the Adobe apps mentioned above, installation on a desktop and a 2d copy on a laptop is within the agreement as long as both copies aren't used simultaneously.
http://www.macintouch.com/officevx10.html - feb27

MACinations March 04