I found this interesting post through Avid Book Reader about Amazon locking down the number of times you can download a Kindle eBook. Good things to know before you make a purchase.
iTunes does this type of limiting function too especially when it comes to the iPhone. You can only access your authorized iTunes account through a limited number of computers. With Apple iTunes though you can also reset that limit and unauthorize all the computers then reauthorize only the computers you now want etc etc etc.
Anyway not unheard of DRM/Authorization traps there. Too bad both of these issues are not told to the customers before they buy products from the company.