"Unlimited" data plans
"Unlimited" data plans are the culprit (smart) phones nowadays lack DUN (Dial Up Networking - I wonder who invented the term "Tethering" - it is stupid!).
The reason is unlimited plans are not really unlimited and a small percentage of DUN users will abuse their data plans to download movies, Linux distros, run p2p, etc on their PCs.
The problem lies with operators - who lie when calling their plans unlimited. This is a matter for FTC, EU Comission & other regulators - operators need to put a monthly cap on their plans (and make it clear to their customers), then DUN will be no more a problem - the 1% who abuse their plans will simply hit the limit and face the consequences as they should (=pay more). The other 99% will happily be able to use DUN.
No DUN is mostly the case with phones "originating" from US or with US based operators (I use "originating", because virtually ALL phones are designed in Taiwan and built in China by companies like HTC, Foxconn, Quanta, Compal,...). To list a few:
Handspring/Palm Treo: Out of the box DUN only works from (Unlocked) Treo 650 onward; older Treos "officially" could not do it - it was however possible to do DUN on Treo180/270/600 wia IR and 650 via BT (Bluetooth) with SW like PdaNet and WModem. I'we got through a lot of pains to make DUN work on Treos 180s, 270, 600 and 650s I we had.
iPhone: no DUN (either USB or Bluetooth)
Android: no "real" DUN (=modem over USB or BT) - only USB DUN works with SW on PC on Unlocked phones
PdaNet (www.junefabrics.com) enables BT DUN profile (no SW needed on PC side )& USB DUN (again with SW on PC side) - I think also for locked phones
But PdaNet is annoying: it costs $20 to enable a basic feature all "normal" GSM phones have had since dawn of time (=DUN profile) and the annoying part is you need to run the PdaNet application on Android phone (I tested on HTC Desire) for the BT DUN profile to work.
AFAIK, all "non US" based phones (SE, Nokia, LG, Samsung) with Bluetooth support the BT DUN profile by default.
I was quite shocked to learn that Android platform, not even in it's latest incarnation (2.1) supports one of the most basic Bluetooth profiles - the BT DUN. This is simply inexcusable!
Lack of out-of-the-box BT DUN was a showstopper for me buying the HTC Desire (which after testing it I, well, desire)!
And to think I used to dream Android (being open) will also support the BT FAX profile (not many phones do, AFAIK) so I could (finally) send faxes from my PC via GSM through BT connection...
I simply hate when progress is not only stopped but goes backwards.
I remember my first GSM (Ericsson GA628 - "the brick" in 1998 - a simple, cheap and wonderful phone) - it had a modem feature by simply connecting it to a PC (or other terminal device) via RS232 and could dial data (or voice) calls through AT commands.