An interesting point to note with the recent opening of the Mac App Store is the terms and conditions for the iTunes Store (which powers the Mac App Store). To note (emphasis mine)
USAGE RULES (i) You shall be authorized to use Products only for personal, noncommercial use. For details of your rights and restrictions on your rights to use the Products, see: www.copyright.org.au; or www.ipaustralia.gov.au/ip/copyright.shtml; or www.copyright.asn.au.
So, that bargain price on Aperture for $100, instead of $250 - not if you’re a pro photographer, only for personal use. That copy of Keynote or Pages that you got for $24 - not if you’re making a presentation for work, you need to buy the retail pack for $130…