While there are MANY social reasons why cell phones shouldn't be on airplanes (many enumerated here in the comments), there are technical reasons why it isn't a good idea.
The "interfering with air navigation" excuse sounds nice, but it really doesn't hold any water. No way reasonable navigation aids (well designed ones) should be susceptible at all. On the other hand, the cell towers are another matter. You see there are a couple of reasons. First, most of the cell tower antennas point horizontal (and a bit downward) to pick up terrestrial based cell phones (well, that's where they are). They don't send their main signal "up to the sky" as it would be wasted. Secondly, cell phones in the sky broadcast (and receive) their signal in an omnidirectional pattern, so in the air they would be in contact with LOTS of cell towers line of sight. Since the cell towers normally signal "outward" not "upward" they need to increase their power to even get to the flying high cell phone, which causes interference with other cell towers, limiting capacity of many towers. In addition, your little cell phone must increase its power in its attempts to communicate as well, and that drains your battery faster.
So, for a bunch of reasons, both technical and social, you should turn off the damn phone and chill out.
I also take notice of the 60 minutes (US News show) piece last Sunday indicating that lots of people are making cell phones "addictive" to get more usage. Maybe that's why people want to use them in planes, they gotta feed their habit. (*SIGH*).