DaveC wrote:
Cell phone usage is restricted on all domestic commercial flights by the FAA,
so the flight attendant keeps telling me.

Nope. The FAA rules have not changed on this, nor have the FCC ones.
You can't use Advanced Mobile Phone Service (i.e., traditional cellular)
airborne. That is an FCC rule.

The FAA rule is just that the airline (i.e., the operator) must
determine when electronic devices are safe to use. The only thing that
has really changed is the "conventional wisdom" on the FAA's part got a
little tighter after some supposed interference issues (primarily blamed
on laptops) a few years back.

Is cell phone usage also restricted on private jets, etc. for similar
reasons?

Yes, all airborne use is prohibited. On cell phones permanently
installed on private jets, there has to be a warning label attached to
prohibit it's use in the air. (FCC rule).

So my question... what is the technical reason for restriction of cell phone
us on commercial flights? Is this reason valid, technically? Or is it simply
an excuse to force anyone who needs to make a call to use the airline's
on-board phones (and pay their outrageous rates)?

Forcing you to use the Airphones is the least of the FAA's or the FCC's
concern.

The real reason is that the cellular phone industry does not want you
to. Their bread and butter is the ground based caller, and their
systems are not designed to work with callers that have some altitude on
the system (there are techncial deficiencies in this that I won't go
into unless you really want to know). As a matter of fact, the cellular
industry fought a company who wanted to share the spectrum
tooth-and-nail over airborne use.

Oddly, this prohibition is NOT written into the rules for the newer
wireless services (PCS, NEXTEL, etc...) that operate outside the AMPS
(800MHz) band. However, most of the carriers with these services still
discourate airborne use.