In a twitter reply by Microsoft Australia (MSAU) today, we were reminded of a Microsoft Office license term that many people aren't aware of. The reply to a tweeted question stated that whilst the Home and Student edition of Microsoft Office is licensed for installation on up to three machines, all other versions of Microsoft Office are in fact licensed for installation on two machines. Specifically, the license terms state:
Extract from the MSLT.
The theory behind this not well-publicized license term is that many of today’s computer-users have both a desktop and a notebook (or other portable device), and thus it would be unjust for Microsoft to try to charge these users for both machines. Of course, a possible second reason for this license term is that more machines with Microsoft's Office installed equals less machines with competing productivity suites installed – and even if users aren't paying for both copies, that's still a win for Microsoft.
This license term applies to all Microsoft Office retail products, including the Academic editions.
Amendment: As iKenndac pointed out in the comments below, other productivity suites may have similar license terms (ie. Adobe products), so be sure to always check your license terms before shelling out for extra copies.