There is something that I don't believe anyone is mentioning here.
While you were watching your show, how many Dell ads came during the commercial breaks? How about HP?
I believe in a lot of cases, if a program is being sponsered with advertising dollars from Company X or Company Y, they are sensitive about overly-glorifying Company Z by putting their products front and center and in your face (and let's face it, there are few logos that are more bold and in my face than a glowing white Apple logo).
I'm sure Dell would not be very happy to pay good money to advertise their Dimension desktop for 15 or 30 seconds, and then have a gleaming beautiful Powerbook shown on the program in actual use by a character. The viewer might say to himself, "hmmm wonder why I should get a dumpy old Dell, when I could use a sexy little Mac like that svelte hipster Drew Carey..."
In movies, or in shows that aren't sponsored by advertising dollars (i.e. Sex and the City), you don't generally see this sort of logo hiding. In shows that rely on their commercial advertisers to pay the bills, you see it quite frequently.